Spécialité: Physique de la Matière...

UNIVERSITÉ DU MAINE - UFR SCIENCES ET TECHNIQUES

École Doctorale de l’Université du Maine

THÈSE

Pour obtenir le grade de Docteur de l’Université du Maine

Spécialité: Physique de la Matière Condensé

Présentée et soutenue publiquement le 18 novembre 2005 par:

Kateryna Fatyeyeva

ELABORATION AND INVESTIGATION OF

CONDUCTING POLYMER COMPOSITES BASED ON

POLYANILINE AND POLYAMIDE

Composition du Jury:

M. A. Bernès Professeur, Université Paul Sabatier de Toulouse Rapporteur

M. D. Graebling Professeur, Université de Pau et des Pays de l’Adour Rapporteur

M. A. Korzhenko Docteur, CERDATO, Arkema, SERQUIGNY Examinateur

M. J.-F. Pilard Professeur, Université du Maine, LE MANS Examinateur

M. A. Pud Docteur (HDR), ICBP, Kiev, UKRAINE Codirecteur de thèse

M. M. Tabellout Maître de Conférence, Université du Maine, LE MANS Directeur de thèse

This thesis was prepared in:

Laboratoire de Physique de l’Etat Condensé Université du Maine, CNRS-UMR 6087

Av. O.Messiaen, 72085 Le Mans cedex 9, France

and

Laboratory of electrochemistry of organic compounds Institute of Bioorganic and Petrochemistry, National Academy of Science of Ukraine

50, Kharkovskoe shosse, 02160 Kiev, Ukraine

email: [email protected]

Acknowledgement

Taking the opportunity I would like to express my gratitude to the people who

have helped me in my work over a few past years.

Special thanks should be given to my supervisors Dr. Alexander Pud (Institute

of Bioorganic chemistry and Petrochemistry (IBCP), Ukraine) and Dr. Mohamed

Tabellout (Université du Maine, France) for their invaluable assistance in performing

this work and discussing the results.

I would also like to thank the committee members including Prof. Alain

Bernès, Prof. Didier Graebling, Dr. Alexander Korzhenko and Prof. Jean-François

Pilard for finding time in their busy schedule for serving in the committee, reviewing

my thesis and offering some valuable suggestions and comments on the work.

I would also like to thank Prof. Galina Shapoval (IBCP, Ukraine) for giving

me an opportunity to begin my research work in her laboratory.

I would like to thank the French government for providing me with the

financial support as a grant during my stay in France.

I am very grateful to the colleagues from Laboratoire de Physique de l’Etat

Condensé (LPEC) (France) as well as from the IBCP (Ukraine) for their help in making

experiments, for their encouragement and kindness. I greatly acknowledge the

assistance of Dr. Pascal Ruello (LPEC) in performing the four-probe resistivity

measurements. I would like to thank Dr. Jean-François Bardeau (LPEC) for assistance

in taking Raman spectrometry measurements, Yann Bulois (LPEC) for the electron

micrographs of the composite films, Gérard Guevelou (Laboratoire Polymères,

Colloides, Interfaces) for performing DSC measurements and Dr. Nikolay Ogurtsov

(IBCP) for his help in performing thermal analysis measurements, Olivier Schneegans

and Frédéric Houzé (Laboratoire de Génie Electrique de Paris, UMR CNRS 8507,

Supélec, Université Paris VI and Paris XI) for performing AFM (electrical and

topographical images) of the composite films. I’m grateful to Pierre-Yves Baillf for his

constant help and kindness. Also, I am appreciating the help of Jeannette Le Moine

(LPEC) very much.

In particular, I would like to thank Arkema company (France) for giving the

polyamide samples.

Special acknowledgement should be directed to Dr. Sergey Rogalsky (IBCP)

and to Dr. Jean-François Bardeau (LPEC) for their helpful discussions and comments

on the work and for their readiness to help.

3

Acknowledgement

I owe my thanks to Karolina Galicka for her assistance and friendship and to

all those who have directly or indirectly helped me in the research work and during my

stay in France.

A giant thank you goes to my parents for their love and patience they showed

in the course of taking this thesis and especially during my study in France. This work

would not be possible without their support. Finally, a big thank you to Andrey for his

support, love and belief in me.

4

Table of contents

Abbreviations…………………………..………………...…………………….... 9

Introduction……………...……………………….….……….………………..…. 11

Chapter 1. Literature review

1.1. Structure and properties of polyaniline…………………......................... 13

1.1.1. Chemical properties………………………………..................... 13

1.1.2. Electrochemical properties……………………………………... 16

1.1.3. Thermal, dielectric and mechanical properties of polyaniline…. 24

1.1.4. Influence of acid-dopants on the polyaniline preparation and

properties…………………………………………………....................

26

1.2. Synthetic methods of the polyaniline preparation………………............ 28

1.2.1. Chemical way of obtaining polyaniline………………………... 28

1.2.2. Electrochemical way and the mechanism of the polyaniline

synthesis...……………………………………………………………..

29

1.2.3. Comparison of chemical and electrochemical methods of the

polyaniline synthesis…………….…………..………………………...

35

1.3. Mechanism of electrical conductivity in polyaniline………………….....36

1.4. Practical application of polyaniline…………………………....................41

1.5. Composite materials based on polyaniline and polyamide………………43

1.6. Research motivation…………………………………………………….. 47

Chapter 2. Experimental part

Introduction…………………………………………………………………...49

2.1. Materials and reagents………………………………………….……….. 49

2.2. Preparation of the composite materials…………………………..............52

2.2.1. Synthesis of polyaniline………..………………………………...52

2.2.2. The formation and pre-treatment of the polymer matrix............... 52

2.2.3. Formation of the surface conductive composites…..…………….54

2.2.4. Formation of the bulk conductive composites……….….............. 56

2.3. The main investigation methods and techniques………...…………...…. 57

2.3.1. Method of determination of the real polyaniline content in the

composites………………….……………………..…….………..……..

58

2.3.2. Electrochemical investigations………….…………………..……60

5

Table of contents

2.3.3. Electrical conductivity measurements..………….…….................63

2.3.4. Spectroscopy investigations…………..…………………………. 65

2.3.5. Dielectric relaxation spectroscopy…………..………….……….. 67

2.3.6. Thermal analysis and differential scanning calorimetric

measurements………..………………………………………..………... 71

2.3.7. Mechanical analysis……………………..………………………. 72

2.3.8. Optical and atomic force microscopies……………..…………… 72

2.3.9. pH-potential-temperature measurements……………................... 72

Chapter 3. Electrochemical synthesis of polyaniline

Introduction………………...………………………………………………… 73

3.1. The peculiarities of the electrochemical formation and stability of

polyaniline on the surface of bare electrodes..……………………...………... 73

3.1.1. Polyaniline formation by cyclic voltammetry……..…….............. 73

3.1.2. Polyaniline formation in the potentiostatic mode……………….. 79

3.1.3. Polyaniline formation under galvanostatic conditions…………... 79

3.1.4. Electrochemical properties of the polyaniline film in the

background solution…….………..……………….................................. 82

3.2. The formation of the polymer composite materials……………………... 86

3.3. Conclusions………………………………..…………………….............. 94

Chapter 4. Chemical aniline polymerization in solid and water dispersed

polyamide media

Introduction……………………………..……………………………………. 96

4.1. The swelling kinetics of the polyamide films in the reaction media.…….96

4.2. The influence of the oxidative media on the polymerisation process…… 99

4.3. Evaluation of the structure of the surface composite films…..………….. 105

4.4. Kinetic peculiarities of the polyaniline formation in the polyamide

matrix…………………..………………………………….…………………. 110

4.4.1. Polymerization process in the PA-12 film………………………. 110

4.4.2. Influence of the polymer matrix structure………………………. 119

4.5. The aniline polymerization in the dispersed polyamide media…............. 125

4.6. Conclusions…………………………..………………………….............. 131

6

Table of contents

Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman

properties

Introduction…………………………………………..…………………….... 134

5.1. Spectroelectrochemical investigation of polyaniline and its composites...134

5.1.1. Spectroelectrochemical properties of the polyaniline films.……..134

5.1.2. Spectroelectrochemical properties of the conducting composite

films……………………………………………………………….….…

136

5.2. Thermal behaviour of polyaniline and its composite materials………….140

5.2.1. Thermal behaviour of polyaniline…….………..………….……..140

5.2.2. Thermal behaviour of the conducting composite powders….…... 142

5.3. Mechanical investigation of the conducting composite films……………147

5.4. Structural studies of the surface conducting composite films……………150

5.5. Raman spectrometry measurements………………………...……………156

5.5.1. Raman spectrometry investigation of polyaniline……..…………156

5.5.2. Raman spectrometry study of the surface composite films……... 159

5.5.3. Raman spectrometry study of the bulk composite powders…….. 162

5.6. Conclusions………………..……………………………………..............166

Chapter 6. Dielectric and electrical properties

Introduction………………………………..………………………………….168

6.1. Chemically synthesized polyaniline………………………..…………….168

6.1.1. Dedoped polyaniline……………………………………..………168

6.1.2. Doped polyaniline…………..……………………………............173

6.2. Surface conducting composite materials………………………………....183

6.2.1. Dielectric properties of the polyamide matrices...……….…........ 183

6.2.2. Dielectric and electrical properties of the surface conducting

composites based on the PA-12 matrix..………………………………..

187

6.2.3. The influence of the polymer matrix structure...………………....192

6.3. Bulk conductive composite materials based on polyaniline and

polyamide……………………………………………………………………..

197

6.3.1. Effect of the dopant nature on properties of the composite PA-

12/PANI powders……………………………………………….............

198

7

Table of contents

6.3.2. Influence of the polyaniline content……………………………...203

6.3.3. Influence of the polyamide matrix structure.……………………. 215

6.3.4. Properties of the composite films…………..…………………….216

6.4. Conclusions…………………………………………..………….............. 226

General conclusions and suggestions for future work…………………................. 229

References……………………………………………..………………………….. 232

8

Abbreviations

13C-NMR – nuclear magnetic resonance

ac-conductivity – alternating-current conductivity

AFM – atomic force microscopy

AMPSA – 2-acrylamido-2-methyl-1-propanesulphonic acid

APS – ammonium persulphate

BeOH – 2-hyroxybutanol

BN – acid eutectic mixture NH4F+2.35 HF

CELT – charge energy limited tunnelling

CSA – camphor sulphonic acid

DBSA – dodecyl benzene sulphonic acid

dc-conductivity – direct-current conductivity

DiBHP – diester [bis-(2-methylpropyl) hydrogen phosphate]

DiOHP – diester [bis-(2-ethylexyl) hydrogen phosphate]

DRS – dielectric relaxation spectroscopy

DSC – differential scanning calorimetry

E – emeraldine

EB – emeraldine base

EPR – electron paramagnetic resonance

EQCM – electrochemical quartz crystal microbalance

ES – emeraldine salt

FIT – fluctuation-induced tunnelling

FTIR-spectroscopy – Fourier transform infrared spectroscopy

HN function – Havriliak-Negami function

ICP – intrinsically conducting polymers

IR – infra red

LE – leucoemeraldine

MSA – methane sulphonic acid

MWS effect – Maxwell-Wagner-Sillars effect

NMA – N-methylaniline

NMP – N-methyl pyrrolidone

NSA – β-naphtalene sulphonic acid

PA – polyamide

PANI – polyaniline

9

Abbreviations

PET – poly(ethylene terephtalate)

PM – polymer matrix

PMMA – poly(methyl methacrylate)

PNA – pernigraniline

POMA – poly(o-methoxyaniline)

PPN – polyphenylnitrenium

PVA – poly(vinyl alcohol)

PVDF – poly(vinylidene fluoride)

SANS – small-angle neutron scattering

SCE – saturated calomel electrode

SHE – standard hydrogen electrode

TEM – transmission electron microscopy

TGA – thermogravimetric analysis

TSA – p-toluene sulphonic acid

UV-Vis spectroscopy – ultraviolet-visible spectroscopy

VRH – variable range hopping

XPS – X-ray Photoelectron Spectroscopy

-N=Q=N- – quinoid diimine unit

-NH-B-NH- – benzenoid diamine unit

10

Introduction

Introduction

Conducting polymer materials are widely used in different fields of science and

industry – as electrodes in rechargeable batteries, as electrochromic displays, anti-

corrosion coatings, electromagnetic interference shielding, sensors, membranes, etc.

Nowadays, two groups of conducting polymer materials exist. The first group

consists of composite materials, which receive their conductivity after introducing

conductive dispersed materials with electronic conductivity (carbon, metallic powder or

fibres) into a polymer. Such materials are called “materials with external conductivity”

– in order to underline that conducting particles were introduced externally. The other

group of conducting polymer materials is a class of materials, conductivity of which is

reached by the motion of charge carriers along the polymer backbone. These materials

were given the name of “intrinsically conducting polymers” (ICP).

Before these materials were discovered (more than 25 years ago), conducting

polymers were obtained only by the cardinal change of the polymer insulators or

semiconductors structure, for example as a result of their heating till the temperature at

which pyrolysis or partial grafitization of the polymer took place.

Conducting polymers are a rather new class of materials which attracts

attention due to the combination of electrical, magnetic properties of metals with optical

property of usual polymers. They receive a high conductivity value during both the

redox and the doping processes, i.e. after the oxidation (reduction) and the introduction

of a necessary quantity of acid (dopant) anions in the polymer matrix (PM) of

conjugated polymers with a conductivity value about 10-15-10-10 S/cm. As a result

materials with semiconducting and metallic conductivity (from 10-9 to 103 S/cm) can be

obtained depending on the method of synthesis, the nature of polymer and dopant. The

typical representatives of this kind of materials are polyacetylene, polyaniline (PANI),

polypyrrole, polythiophene and their derivatives.

Major works dealing with conducting polymers are devoted to the problems of

their synthesis and possible applications [1-5]. In 2000 Hideki Shirakawa, Alan G.

MacDiarmid and Alan J. Heeger, who discovered the existence of conducting polymers,

were awarded by the Nobel Prize in Chemistry.

PANI is the oldest polymers among the known ICP. It was synthesized for the

first time as early as 1862 [1]. PANI takes a special place among other polymers due to

the unique combination of easy synthesis, low monomer (aniline) cost, high value of

conductivity, high stability of both doped and dedoped states in the air with sensor,

11

Introduction

optical, catalytical and other properties. PANI is a conjugated polymer with an electrical

conductivity value similar to semiconductors.

But there are some limitations in the PANI application, which are connected

with its poor solubility in common organic solvents, its infusibility and fragility. These

disadvantages often hinder PANI practical applications. Some ways to overcome these

problems such as doping PANI by functionalized acids (such as camphor sulphonic acid

(CSA), dodecyl benzene sulphonic acid (DBSA)), using substituted PANI and forming

composites with conventional polymers have been found. From our point of view, the

last way, i.e. the preparation of conducting polymer composites, is the most promising,

as in this case we obtain a material, which combines mechanical properties of common

polymers with the electrical conductivity of PANI.

So, the purpose of this work is to obtain new composite materials by

combining PANI and traditional thermoplastic polymers - polyamides in order to take

advantage of the properties of both kinds of materials. Two kinds of such composites –

surface layered and bulk – are synthesized and investigated by standard techniques.

The first chapter of this thesis is devoted to the literature review of the

conducting polymers, especially of pure PANI and composite materials based on

polyamide (PA) and PANI. The used materials as well as the composite preparation

methods are expounded in the second chapter. The description of the methods used in

the course of the investigation are also given. In the third chapter the peculiarities and

the mechanism of the aniline electrochemical polymerization on the bare electrodes and

on the electrodes covered with polymer matrices are stated. The fourth chapter of this

thesis is devoted to the chemical aniline polymerization process inside the PA films and

in the presence of the water PA dispersions. The kinetic features of the process are

described. The fifth chapter deals with the spectroelectrochemical, thermal, mechanical,

structural and Raman properties of the obtained surface and bulk conducting composite

materials. It was confirmed that in the course of forming the composite materials the

interaction between PANI and the PM takes place. The sixth chapter describes the

dielectric and electrical properties of the composite materials. Large frequency range

(10-1–109 Hz) allows revealing different relaxation processes in PANI as well as in the

composite conducting materials. Percolation theory is used to analyze the obtained

experimental results.

12

Chapter 1

LITERATURE REVIEW


1.1. Structure and properties of polyaniline

1.1.1. Chemical properties

In a broad sense the name of PANI combines a family of polymers (Fig. 1.1),

which have the same length of the polymer chain, but differ in the oxidation state, redox

properties and the value of conductivity. It is accepted that PANI exists in three well

defined oxidation states: leucoemeraldine (LE), emeraldine (E) and pernigraniline

(PNA) (Fig. 1.1). LE and PNA are the fully reduced (all the nitrogen atoms are amine)

and the fully oxidized (all the nitrogen atoms are imine) forms, respectively, and in E

the ratio amine/imine is ∼0.5. Starting from the electrically insulating LE, electrically

conducting E can be obtained by standard electrochemical or chemical oxidation, as for

all other conducting polymers [6]. But upon further oxidation a second redox process

occurs, which yields a new insulating material - PNA. In addition to this unusual

behaviour, a decrease of conductivity by ten orders of magnitude is obtained just by

treatment of the conducting E in neutral or alkaline media. Protonation induces an

insulator-to-conductor transition, while the number of π-electrons in the chain remains

constant (Fig. 1.2).

Figure 1.1. Structures of the non-conducting PANI forms

N N

x

N

H

N

H H H

NN

x

N

H

N

H

y

NN

x

N N

1-y

Leucoemeraldine (y=1, completely reduced form)

Emeraldine base (y=0.5)

Pernigraniline (y=0, completely oxidized form)

13


Figure 1.2. Illustration of the transition between the PANI base form and the protonated

emeraldine form (one of the possible chemical representations). A- represents an

arbitrary anion

For a long time the PANI structure was arguable and only recently for its

emeraldine base form it was established as poly(p-phenyleneamine quinonediimine)

(Fig. 1.3) [7]. For this purpose the synthesis of the compound of ascribed structure was

made and the following comparison of its properties with the properties of PANI

received by usual methods (by the chemical and electrochemical aniline

polycondensation) was carried out [7, 8].

Figure 1.3. Structure of the poly(p-phenyleneamine quinonediimine)

The PANI structure (Fig. 1.3) was also confirmed by the 13С-NMR [9], IR and

Raman spectroscopy experimental data [10, 11]. In accordance with these results, bands

which are typical for the vibrations of benzenoid and quinoid rings are observed.

Moreover, their ratio depends on the oxidation state, C=N vibrational mode, C-N

bending mode. The type of the C-H out-of plane bending vibration of benzenoid groups

is characteristic of a para-substitution of the aromatic ring. However, Genies et al [8]

paid attention to the fact that in IR-spectra of the specially synthesized poly-о-, p-

phenylenediamine essential differences from the PANI spectrum are present.

NN

x

N

H

N

H

y

1-y

NN

x

N

H

N

H

x

N N N N

H H H H

-

Emeraldine base (EB) (blue)

2xHA

+. +.

A- A-

Emeraldine salt (ES) (green)

+2xHA

14


The theoretical calculations (extended Hückel theory) of the planar sandwich

geometry of the aniline dimer cation (С6Н5NH2)+2 [12] showed that the optimal

geometry of this cation is head-to-tail coupling. Moreover, in such conformation the

effective overlap comes mainly from benzene rings and carbon atom interacts with the

corresponding atom from the other fragment (Fig. 1.4).

Figure 1.4. Schematic diagram showing the configuration considered in (С6Н5NH2)+2

[12]

The chemical or electrochemical PANI oxidation leads to the augmentation of

the part of quinoid diimine units and, accordingly, to the decreasing of the part of

benzenoid diamine units. Analytical results obtained earlier [13] and results of the

FTIR-spectroscopy [14, 15] testify to this fact. The deep oxidation of PANI leads to

almost quantitative formation of p-benzoquinone [16]. PANI can be reduced

electrochemically or chemically, for example, by hydrazine, to colourless

polyphenylamine, which does not contain quinoid diimine chromophore groups [16].

Due to the presence of basic amine and imine nitrogen atoms, the polymer

reacts with protonic acids, forming the PANI salts. PANI in the state of the salt is

usually formed during the synthesis in an acidic solution. Interaction of these salts with

alkaline or ammonium solutions converts PANI into the base form (Fig. 1.2). It was

established that only 50% of nitrogen atoms could be protonated [17] that was explained

by the existence of strong effective pushing away between protons near to neighbouring

nitrogen atoms [18].

PANI-base is partly soluble in some polar organic solvents, such as aromatic

amines, phenols, cold pyridine, N,N-dimethylformamide [13], and also in aqueous

solutions of acids: in cold 80% acetic acid, 60-88% formic acid [13, 16]. The molecular

weight of the soluble in tetrahydrofuran fraction of PANI-base, which was

electrochemically obtained in aqueous solution, changes in wide ranges – minimum

established value is 4300 [13], and maximum - ~50000-55000 [19]. These

NH2

H2N

.+

15


disagreements can be explained by differences of synthesizing methods and methods of

treatments resulted in the molecular-weight distribution shifts to the low molecular

weight side [20]. Also, it was found [21] that the temperature of the synthesis

influenced the molecular weight: as the polymerization temperature was decreased, the

molecular weight of PANI increased. However, the PANI molecular weight was not

affected by the acidity of the reaction medium. The soluble in N,N-dimethylformamide

the PANI fraction, which was synthesized in the acid eutectic mixture NH4F + 2.35 HF

(the medium abbreviation is BN), has molecular weight about 80000-90000 [22]. The

PANI salts with inorganic acids are soluble only in concentrated sulphuric acid [10].

However, using organic proton acids of large molecular size makes the PANI salts

partly soluble also in common organic solvents [23].

It is known that PANI can exist in several oxidation states. The protonation-

deprotonation and oxidation-reduction processes do not change the PANI chain and are

practically reversible. That’s why it was proposed [24, 25] to depict the relation

between these states as a two-dimensional potential-pH diagram. However, a point

location in this “external” diagram does not give directly the PANI structure in this

point. For theoretical analysis of transformation processes with the participation of the

different PANI forms the system with the coordinates of the electronic band filling (fe),

0<fe<1, and the location of the available sites which are protonated (fp), 0<fp<2, is more

convenient [26]:

fp = ne / nN; fе = (nN + np - ne) / 2nN

where ne, np, nN are amount of electrons, protons, and nitrogen’s, respectively.

The full diagram of the PANI states in “internal” coordinates fp, fe, which

includes all of the possible PANI forms in head-to-tail coupling is depicted in Fig. 1.5

[16], where -N=Q=N- - quinoid diimine units, and -NН-В-NН- - benzenoid diamine

units.

1.1.2. Electrochemical properties

PANI displays electroactivity only in acidic media [27]. The PANI

electroactivity is associated with the polymer transition between salt and base forms

(Fig. 1.2). In organic solvents PANI also reveals electroactivity, but only in the presence

of protonic acid as well as organic salt as electrolyte [28].

The PANI tendency to redox transitions was found with the help of cyclic

voltammograms [29]. If the PANI form displays electroactivity, one can see current

(1.1)

16


Figure 1.5. State diagram of the theoretically possible PANI forms:

1 – -В-NH-B-NH-B-NH2-, LE-base;

2 – -B-N+Н2-B-N+Н2-B-N+Н2-, protonated LE;

3 – -B-N=Q=N-B-NH-B-NH-, Е-base;

4 – -B-N+Н=Q=N+Н-B-NН-B-NH-, protonated Е, salt form;

5 – -B-N=Q=N-B-N=Q=N-, PNA-base;

6 – -B-N+Н=Q=N+Н-B-N+Н=Q=N+Н-, protonated PNA;

7 – -B-:N+-B-:N+-B-:N+-В-:N+-, or =Q=N+=Q=N+=Q=N+=Q=N+-, polyphenylnitrenium (PPN)

peaks on the cyclic voltammograms. The height and potential of these peaks will

depend on many factors. Cyclic voltammograms of PANI, which were obtained on the

platinum electrode in the acidic aniline solution depicted in Fig. 1.6, have three anodic

and three cathodic peaks. During the analysis of the electrochemical PANI behaviour

only couples 1-1’ and 3-3’ were usually considered, since the couple 2-2’ is not

specifically associated to PANI [29]. Even in the case of presence of this redox couple

on the polymer cyclic voltammograms, its behaviour does not depend on the other two

redox peaks, which are connected with each other as we can see from the results of

chronocoulometric study during the synthesis in the BN media [30]. There aren’t any

peculiarities on EPR spectra which may correspond to the couple 2-2’ in contrast to the

couples 1-1’ and 3-3’ [31].

There isn’t a generally accepted explanation of the reason of the emergence of

this middle peak couple 2-2’ [32-36]. For example, Park S.-M. et al [33] have supposed

that in the aqueous acidic solution this peak is connected with the redox reactions of the

17


Figure 1.6. Multisweep cyclic voltammograms during

deposition of PANI in 0.5 M H2SO4 containing 0.5 M

aniline [34]. Potential vs saturated calomel electrode

(SCE). Electrode – Pt.

PANI degradation which are accompanied with formation of quinoneimines and p-

benzoquinones. Genies E.M. et al [34] have demonstrated that the middle peak which

often appears during potential cycling between -0.2 and 1.2 V (vs Cu/CuF2) in BN, is

attributed to the presence of polymer containing phenazine rings. This peak appears

during the polymerization of aniline at high potential and also can be a result of

oxidation of the formed polymer at a higher potential. Yoneyama H. et al [32] have

demonstrated the conformity of couple 2-2’ to the redox reactions of p-benzoquinone

which they found with the help of spectroelectrochemical measurements. The

appearance of this couple means the beginning of the loss of the PANI electroactivity

that is observed at the PANI overoxidation. In aqueous solution such degradation, as

studied by rotating ring-disk electrode [33], leads to the formation of soluble products.

They may be a quinone/hydroquinone couple [37]. On the other hand, in the BN media

the couple 2-2’ is connected with the PANI cross-linking [34]. On the basis of

theoretical discussions, the authors [38] assumed that in the range of potential of the

couple 2-2’ the electroactive PANI oligomers with the conjugation length less than 10

aniline units can be formed.

The models of the redox PANI transitions (corresponding to the couples of

peaks 1-1’ and 3-3’) proposed in literature can be presented in the form of the schemes

(1.2) and (1.3).

On first sight, it seems that we can determine the PANI oxidation state at each

potential with the help of coulometric method on the basis of LE, colourless of which

has been already indicated by the absence of chromophore quinone diimine groups [16].

Existing results show [38] that for the complete reduction of emeraldine to LE we need

18


about 0.5 ē per aniline unit, and the comparison of results [31] and [39] allows m

the conclusion that for the LE-PNA transition we need one electron per aniline

accordance with the scheme (1.2). However, the results of coulometric study [30]

give reason for such a simplified interpretation. The interpretation of coulo

measurements in the case of conducting polymers becomes more complica

significant effect of capacitive currents [40]. The study of the impedance-fre

dependencies indicates that the storage up to 40% of the PANI charge in pro

carbonate electrolyte takes place due to the capacitive current [41]. Such ef

significant also in the aqueous media [42, 43]. So, to explain redox transitions in

it is necessary to have additional information.

The most substantiated is the following variant of scheme (1.2) of the

redox transitions (scheme (1.4)). At рН > 4 PANI is non electroactive.

The redox potential-pH dependency is determined in accordance with

equation (1.5).

The potential of peak 1 (Fig. 1.6) in weak acidic solution does not dep

pH, but in strong acidic solution its value shifts to the anodic region by 59 mV at

decreasing by a unit [34]. Consequently, in weak acidic media protons do not ta

in the reaction and as a result the potential for the first redox process is indepen

pH. In contrary, in strong acidic media the relation of one Н+ to electron occu

analogy with this, in weak acidic solution, in which the study of the potential

)

1:- 2e

1':+ 2e 3':+ 2e

3:- 2e

-

--

NH+

NH NH.. ..

NH+

-

3':+ 2e

3:- 2e-

-++NN

LE PNA

PPN

NH NH.. ..

- NH NH.. +

. 3:- e

3':+ e1':+ e

1:- e

-

-3:- e

3':+ eNH+

NH+

LE E

PNA

19

(1.3)

q

k

d

(1.2

aking

ring in

do not

metric

ted by

uency

pylene

fect is

PANI

PANI

Nernst

end on

the pH

e part

ent of

rs. By

of the


NH B NH B

.. +NH B NH B

- e, + A- -

A-

- e, - 2H , - A- -+

BN.. ..

Q N

NH B B

+

+

..

NH2

A-

--e, - H

+NH B

-

NHNH

NH.

BA

A ABQ

- e, + A--

+

- -

couple 1 - 1'

couple 3 - 3'

Ox + mH+ + nē → Red,

Е = Е0 + (0.059/n) lg(Ox/Red) – 0.059(m/n)pH (at Т = 298 К)

peak 3 of the pH dependency was successfully performed (the polymer degradation

makes it difficult), the shift of the potential by 118 mV by pH unit makes the

participation of two Н+ per electron possible [27, 44]. The possibility of the using PANI

as pH-sensitive electrode-sensor is based on its potential-pH dependency [45, 46].

The anion migrations during redox transitions of PANI were studied in

sulphuric acid solutions on the basis of changes of the electrode weight by the

electrochemical quartz crystal microbalance (EQCM) technique [47, 48] and with using

the sulphuric acid with marked 35S by a radiotracer method [49], and also in the solution

of perchloric acid [50]. The obtained results are in good agreement with the scheme

(1.4). The anion desorption at the potentials of peak 3 which was noted in these works is

particularly essential for proving the PANI oxidation mechanism.

The above presented data exclude the possibility of the PANI oxidation more

than PNA, at least in aqueous media at pH > 0. The investigation of the second stage of

the PANI oxidation in strong acidic media is difficult because of the fast polymer

degradation.

The scheme (1.4) lets us to suppose that the redox properties of PANI-base in

the intermediate oxidation emeraldine state are stronger than in more reduced and more

oxidized states. This fact is also confirmed in [27].

The phenomenon of electrochromism, i.e. the change of electronic spectrum

and the colour depending on electrode potential and, correspondingly, on the oxidation

state (Fig. 1.1 and 1.2), is typical for PANI, like for other conducting polymers [14, 48,

(1.5)

(1.4)

pH = 1 - 4 pH < 0

20


51-53]. In reduced LE state, as it was pointed out above, PANI is colourless as the

electron transition at approximately 300 nm, i.e. in UV-region, is observed. This

transition is preserved in all oxidation states of PANI and interpreted as excitation from

valence band to conduction one. This excitation is also responsible for the π-π*

transition in aniline [54]. In the intermediate emeraldine salt oxidation state two

additional peaks appear in UV-Vis region, at approximately 400 and 800 nm, but in the

fully oxidized state instead of these peaks a broad band at approximately 600 nm is

observed. The colour of PANI during this transition changes from green to blue. The

analogous change of UV-Vis spectra is observed during the transition from substituted

phenylenediamines to their cation-radicals and to appropriate quinone diimines:

The authors [55-57] paid attention to this fact and used it for the interpre

of the PANI redox transitions. Besides, the two transitions in the spectru

emeraldine salt are in good agreement with the predicted transitions for pol

structure of PANI [58], and the transition in fully oxidized state corresponds

predicted formation of the molecular exiton [59, 60] for which the presence

quinone diimine units is necessary.

The PANI structure changes were investigated in situ by FTIR-spectrosco

HCl aqueous solution with different рН values [11]. The oxidation from LE with

first peak on the cyclic voltammogram leads to the decreasing of N-H stre

vibrations intensity and to the transition from benzenoid structure to partially qu

one. Essential changes in the intensity which would be caused by the vibratio

anions do not take place. These facts allow interpreting the first oxidative proces

oxidation connected with the deprotonation, but without the exit of anions.

interpretation agrees with “strong acid” variant of the scheme. Further PANI oxi

(peak 3) leads to the increase of the intensity of N-Н vibration bands [11]. This a

adding to the scheme (1.4) the oxidation reaction of the emeraldine salt with the en

anions to the PNA salt. It is necessary to do this as the deeper oxidation will requi

process of deprotonation in accordance with the scheme (1.3).

However, in organic media the redox mechanism of PANI does not inclu

proton exchange with solution on which the possibility of multi-sweep cycling o

polymer in the system with lithium counter electrode indicates [1]. Althoug

conclusion contradicts to some of the given above results. The facts of monot

NH B NH B BBB NH B NHB +.B N Q N

21

(1.6)

tation

m of

aronic

to the

of the

py in

in the

tching

inoid

ns of

s like

This

dation

llows

try of

re the

de the

f this

h this

onous


growth and decreasing of the electrode mass during oxidation and reduction,

respectively, in the LiClО4/propylene carbonate electrolyte presuppose the participation

of only anions in the PANI redox transitions [61]. These data explain the mechanism

which combines the left top and right bottom reactions of the scheme (1.4) for the aprotonic solution [57]. It should be noted that in such media non protonated emeraldine

also exhibits electroactivity which can be described by the following reaction [55]:

The electrochemical PANI transitions are accompanied by changes

conductivity. As it is shown by in situ carried resistance measurements on direct c

in aqueous [56, 62] and nonaqueous (propylene carbonate) [61] electrolytes a

impedance measurements on alternating current in aqueous solution [63] the cond

PANI form is only the intermediate oxidative state of protonated PANI, i

emeraldine salt (Fig. 1.2). Beginning at certain potential, both reduction and oxi

of this form of PANI lead to the sharp growth of electric resistance. Cathod

anodic limits of the existence of the conducting PANI form are potentials of peak

and 3-3’, respectively: these limits are being shifted at changes of pH solution to

with the peaks on the cyclic voltammograms (Fig. 1.7).

Figure 1.7. Influence of pH on the PANI resist

(Е = 0.35 V vs SCE, рН 1) [44].

рН: 1 – 1.0; 2 – 2.6; 3 – 3.6; 4 – 4.4; 5 – 5.0

The PANI conductivity in reduced and oxidized states can be determine

electrochemical data: oxidation and reduction of compounds from the solution

electrode surface covered with PANI occur only in the cases when these reaction

NH B B

NH

NH BN Q N

..+ +

- 2e+ 2A

--

--BNH BN Q N

AAQ

..

22

(1.7)

of its

urrent

nd by

ucting

.e. the

dation

ic and

s 1-1’

gether

ivity

.

d from

on the

s take


place in the potential range of polymer electroactivity [30]. The charge transfer through

PANI in a wider potential range (at potentials less than -0.2 V and more than 0.8 V vs

SCE) [64] were not confirmed in the mentioned work [30]. Due to the presence of conductivity in limited range of potentials PANI was called a “chemical diode” [65] and

this property was used for creating the light-emitting device [66].

Besides conductivity, magnetic properties of PANI also change during the

redox PANI transitions: the maximums of intensity of EPR signal are in agreement with

the current peaks on cyclic voltammograms [67-69] (Fig. 1.8). In addition, the character

of magnetic sensitivity changes. These phenomena find satisfactory explanation within

the limits of the scheme (1.4).

Arguments in favour of the scheme (1.3) of the redox PANI transitions have

been reported in the literature [6, 22]. However, the scheme (1.3) cannot be accepted as

basic for the PANI redox transitions, since in some cases the schemes (1.2) and (1.4) are

realized too.

Also, it should be taken into account that the PANI electrochemical behaviour

(i.e. the form of cyclic voltammograms) in aqueous media significantly depends on the

acid anion which is present in the solution [64, 70-72].

Figure 1.8. Cyclic voltammogram (1);

EPR absorption and injection current

(2) for the PANI film on a platinum

electrode in 0.5 M H2SO4. Potential

was scanned between -0.2V and 0.8 V

vs SCE at 10 mV/s [67]

The results considered above show that the process of the aniline

polymerization is rather a complicated one, and runs through many stages. The rate of

each stage depends on many factors (pH of the solution, potential, temperature, solvent,

etc.). It should be also noted that all given above schemes do not take into account the

nature of the acid anion.

23


1.1.3. Thermal, dielectric and mechanical properties of polyaniline

Thermal properties. Determination of optimal processing conditions, under

which proper conductivity and physical properties are allowed, is the most important for

the PANI applications. To study the PANI processing behaviour and to select the

suitable materials for the application of a given polymer it is necessary to take into

account its dimensional stability and thermal history [73]. These properties - thermal

stability and degradation behaviours – are useful in modifying polymers for new

applications. Investigations of the thermal stability of conducting PANI are, hence, of

great importance. But the question of the PANI thermal stability hasn’t been given

enough consideration up to now.

Several researchers [73-76] have studied thermal stability of PANI in both

conducting and insulating forms by thermogravimetric analysis (TGA) and differential

scanning calorimetry (DSC). The typical thermogram of PANI shows a three-step

weight loss process. The first step occurring in the range of 65-125 0C is attributed to

the evaporation of water or solvent molecules from the polymer [73]. The second-step

weight loss occurs between 125-350 0C and is due to the loss of low molecular weight

polymer and unbounded dopant ions from a PANI chain [74]. The third-step weight loss

occurs between 350 and 520 0C and is due to the degradation of the main PANI chain

after the elimination of bounded dopant [75]. But it should be mentioned that the

thermal stability of doped PANI is dependent on the counter anion. For example,

methane sulfonic acid (MSA) doped PANI was found to be stable up to 250 0C [70].

Dielectric properties. Only few articles deal with the dielectric properties of

the conducting polymers although the dielectric function ε*(ω) can provide information

about transport mechanism in the system. But a few studies which can be found in the

literature show that the response of PANI to the electric field depends on many factors

(temperature, electric field frequency, acid doping, water content) [77-80].

The dielectric response is generally described by the complex permittivity

(1.8), where real ε’(ω) and imaginary ε”(ω) components are the storage and loss of

energy in each cycle of applied electric field.

ε*(ω) = ε’(ω) - iε”(ω)

It was found that the values of ε’(ω) were very high at low frequency an

temperature but they are relatively constant at high frequency [79]. Such high

)

24

(1.8
d high
values


may be due to the interfacial effects.

The increase of the dielectric permittivity with doping is a result of

contributions from the backbone emeraldine base and the formed polaron and

bipolarons to the polarization [79]. During the study of ac-conductivity and dielectric

permittivity of emeraldine base and doped PANI it was found that at frequencies below 100 Hz for y = 0.07 (Fig. 1.1) the dielectric permittivity is seen to increase much more

rapidly as compared with y = 0 and y = 0.03 [80]. This fact can suggest stronger

coupling between polarons. It was revealed the presence of the conductivity relaxation

with the help of the dielectric modulus representation M*(ω). Thus, it was shown that

the increase of the dopant concentration leads to an increase in the distribution of

relaxation times [79] and the impedance strength decreases while the relaxation times

become shorter. So, it is concluded that this relaxation times behaviour suggests

multiple paths for the system to relax due to the diffusion of polaron and bipolarons

[80].

Also, it was discovered that dielectric properties of PANI give a good

agreement with the physicochemical parameters which are correlated to statistical

distribution of insulating and conductive segments along the chain [78].

Mechanical properties. As it is known, the main disadvantages of PANI are

its poor processability and mechanical properties caused by its backbone stiffness. It

was found that the mechanical property of PANI can be substantially enhanced when it

was prepared from the gel and after the crosslinking [81, 82]. The influence of the PANI

oxidation state on the mechanical property was also established [81]. It was found that

the tensile strength of EB film was improved by about 25% upon heating to 150 0C.

This improvement was attributed to the interchain crosslinking of PANI at high

temperature and the formation of the PANI aggregates through interchain hydrogen

bonding. The tensile strength of LE improved only upon its exposure to air at 200 0C,

but, at the same time, the colour of the film changed from yellow to blue [81]. The XPS

results revealed the conversion of amine units to imine ones at this temperature [83].

Also, with the help of XPS analysis it was found that the mechanical property of the

PANI film is closely related to the oxidation state of the polymer and the degree of

crosslinking [83].

The film orientation also was found to influence the mechanical properties.

25


Monkman et al [83] showed that the tensile strength increases with elongation which

indicates the alignment of polymer chains along the stretch direction. X-ray diffraction

analysis suggested [82] a possible link of the mechanical behaviour and changes in

crystallinity with elongation.

1.1.4. Influence of acid-dopants on the polyaniline preparation and

properties

The rate of polymerization as well as conductivity and others properties of

PANI strongly depend on the used acid-dopant [84-86]. It is known that the electrical

conductivity is influenced by the size and shape of the counter ion [70].

On one hand, higher the penetration of counter ions in the formed film is, the

higher its conductivity is. But, on the other hand, the small molecular weight anions are

more quickly removed from the polymer. That’s why the choice of dopant might be a

very important factor – if we need thermally stable conducting PANI we should use

larger molecular weight acids, if the high value of conductivity is more important – it is

better to use small protonic anions. The size of counter ion can also affect the interchain

distance and, correspondingly, intermolecular interaction that can change the disorder in

the polymer system [87]. Therefore, the choice of the dopant depends on the PANI

application.

It should be mentioned that the rate of the aniline polymerization also changes

depending on the acid in such sequence: HCl < HClO4 < HBF4 < HF < H3PO4 < H2SO4

[86, 88]. The acid nature at the same doping level determines also the PANI

conductivity as one can see from the results in Table 1.1.

Therefore, it is shown the same anion differently influences the rate of

polymerization and the conductivity value of formed PANI.

In recent studies of PANI doped with various protonic acids [70, 89, 90], it was

found that hydrochloric and sulphuric acids are the best dopants in terms of stability of

conductivity. Kiattibutr et al [91] producing sensors for SO2-N2 mixtures found that at

the same doping level specific conductivity of PANI-HCl is greater as compared with

PANI-CSA because of two factors: a more closely packed crystalline mobility, and the

ability to absorb more water molecules which induced ionic conductivity.

But, in the other work [92] the opposite results can be found – the authors

showed that poly(o-methoxyaniline) (POMA) doped with HCl is easily

26


Table 1.1. – Influence of the anion nature on the PANI conductivity [86]

Anion Doping level Conductivity, Ohm-1⋅cm-1

CF3COO- 0.33 0.7

BF4- 0.34 0.8

ClO4- 0.31 2.0

Cl- 0.36 1.4

NO3- 0.33 1.9

SO42- 0.23 0.5

deprotonated in comparison to polymer doped by p-toluene sulphonic acid (TSA). The

Cl- anion is a weaker base than p-toluene sulphonate, thus, Cl- ions are more easily

removed [92].

Also, amic acids were found to be dopants for PANI [93]. PANI reacted with

the selected amic acid by mixing the N-methyl pyrrolidone (NMP) solutions of the two

materials in appropriate ratios. It was found that the conductivity in all the polymer

matrices was limited by geometric limitations between the two polymers.

Phosphoric acid diesters with long alkyl substituents are also very good

candidates for protonating agents which induce solution and melt processability of

conducting PANI [94]. Due to their lower pK value, compared to the parent acid, they

are sufficiently acidic to protonate emeraldine base. Their hydrophobic alkyl chains also

induce solubility of PANI. Phosphoric acid esters are known as plasticizers for a variety

of polymers. Thus, the esters can serve as plasticizers and protonating agents for PANI.

Protonation of PANI was achieved by treatment with diester [bis-(2-ethylhexyl)

hydrogen phosphate] (DiOHP) and diester [bis-(2-methylpropyl) hydrogen phosphate]

(DiBHP) dissolved in an appropriate solvent (toluene, decaline, chlorinated

hydrocarbons, m-cresol) or by mechanical mixing of PANI with neat diester.

Phosphoric acid esters are also known to form strong intramolecular H-bonds [94].

The PANI-dopant interaction can be related to an increase in the polymer

molecular weight, the polymer crystallinity and/or the molecular conformation of doped

PANI that changes from a compact coil to an open coil-like structure.

Not only conductivity, but also electrochromic properties depend strongly on

the acid-dopant. Gazotti et al [92] established that POMA doped with TSA presents a

higher optical contrast in the visible region than the same polymer doped by HCl.

27


Many researchers also reported that the dopant mixtures induced high

conductivity and processability [90, 95]. Dopant mixtures gave higher conductivity to

PANI than a single dopant, because they could provide an effective conjugation length

and a high protonated level.

It should also be pointed out that not only dopant can influence the PANI

properties, but also the used solvent. As it was shown, the conductivity of the PANI

samples varies with dopants (TSA, CSA, benzene sulphonic and naphtalene sulphonic

(NSA) acids) in the same solvent [95]. However, using different solvents (m-cresol,

chloroform, 2-hyroxybutanol (BeOH)) conductivity with the same dopant dramatically

changes [95], which indicates that transport properties can be controlled by both

dopants and solvents.

Taking into account a specificity of used systems, the observed divergences in

obtained experimental results may be explained by using different polymerization

methods and conditions. Significant fact may have been here also attributed to the

different PANI-dopant interactions.

1.2. Synthetic methods of the polyaniline preparation

PANI is synthesized by the chemical or electrochemical oxidative

polymerization of aniline [6]. There exists also a method of plasma polymerization [96],

but this method is not very convenient and not easy in application. So, we will speak

only about two ways of the PANI obtaining – chemical and electrochemical

polymerization.

It is possible to select such conditions under which PANI is the main reaction

product. In this case the chemical and electrochemical ways of the PANI preparation

produce polymers of the same composition since their electrochemical behaviour is

practically identical [27, 97-99].

1.2.1. Chemical way of obtaining polyaniline

The chemical methods of the aniline polymerization are based on the aniline

oxidation using suitable oxidants such as ammonium persulphate (APS) (NH4)2S2O8,

sodium chlorate NаСlO3, potassium dichromate К2Сг207, Fenton reagent, hydrogen

peroxide H2O2, etc. [99-103] in solutions containing mineral or organic acids. As a

28


result, dark green product is obtained, which because of its colour received the name

“emeraldine”. The treatment of this product by alkaline or ammonium solution converts

it into dark blue emeraldine base (Fig. 1.2). Under these conditions emeraldine can be

received quantitatively: if oxidant is taken on the basis of 1.25 mol per one aniline

molecule, so the yield reaches 97% [16]. Rather similar values have been found recently

[84]: 1.15 ± 0.04 mol of APS per mol of aniline. These results are in good agreement

with the emeraldine formula proposed on the basis of elemental analysis and

quantitative ratio [22] including quinoneimine units 1/4, brutto formula С6Н4.5N:

С6Н5NН2 + 1.25 [О] → С6Н4.5N + 1.25 Н2О.

This formula is, certainly, approximate, but is close enough to reality.

The aniline oxidation reaction is an exothermal reaction with an induction

period [104]. It was established that the presence of PANI leads to the decrease of

induction period time, i.e. the chemical oxidation as well as electrochemical one (look

part 1.2.2) is an autocatalytic process.

The properties of formed PANI are practically independent of the chosen

oxidant, but only under conditions when the oxidant is not taken in excess. In the case

of oxidant excess the decrease of the PANI yield is observed which is due to the PANI

decomposition [84]. Moreover, the polymer formed is a little more oxidized and as a

result the value of its conductivity is lower [84].

1.2.2. Electrochemical way and the mechanism of the polyaniline synthesis

Like many other conducting polymers, PANI can be easily obtained in the film

form on the electrode surface during anodic oxidation of the monomer solution at the

appropriate potentials. Formation of the film with high adhesion to the electrode is

essential for the PANI use in electronic devices and electrochemistry [105].

More often PANI is obtained from the aniline solution in aqueous 0.1-2.0 М

acid solutions as in this media polymer is the main reaction product. The beginning of

oxidation is observed at 0.45-0.65 V (vs SCE) on platinum electrode [64, 89, 106]. A lot

of articles have also been devoted to the PANI electrochemical synthesis in nonaqueous

electrolytes [22, 107-109]. The mechanisms of the initial oxidation have been proposed

[6, 60]. As it has been found from the EPR measurements in the BN media, the

intermediate nature depends on the potentials used [22]. Thus, at 0.7 V (vs Сu/СuF2) the

intermediate is paramagnetic, it is considered to be С6Н5NH2+., whereas at + 1.0 V it is

(1.9)

29


diamagnetic and on the basis of experiments authors [108] have supposed that it must

be the nitrenium cation (С6Н5NH+). The properties of the obtained polymer also depend

on the maximum value of anode potential (Fig. 1.9).

Figure 1.9. Cyclic voltammograms of PANI synthesized at high (а) [29] and

low (b) [47] potentials (vs SCE). Media – aqueous solution of sulphuric acid 0.24 М (а),

рН 1.75 (b). Sweep potential rate 50 mV/s

The process of the PANI formation is characterized by three couples of peaks

which were displayed on cyclic voltammograms in the case of the high anode potential

limit, which is striving for augmentation of the polymer growth rate (Fig. 1.9а). Such

product will be obtained in aqueous as well as in the BN media. However, at lower

potential couple of peaks 2-2’ is absent (Fig. 1.9b) in both aqueous [27, 44] and BN

[108] electrolytes. The limitation of the current densities leads to the same effect [28,

32]. Only in solutions of aqueous hydrofluoric and trifluoro acetic acids it is impossible

to avoid this couple [34]. The couple of peaks 2-2’ can be connected with intermediate

processes, including the cation radical formation in polymer volume as well as the

degradation of the formed polymer. An absence of this couple on the cyclic

voltammogram of previously chemically synthesized PANI [27] can be associated with

careful polymer washing from soluble impurities before the electrochemical study. The

aniline polymerization can take place with noticeable rate at much lower potentials

beginning at 0.55 V (vs SCE) after short polarization at 1 V (vs SCE) which leads to the

formation of the primary PANI film layer [110]. The peak of the aniline oxidation

completely disappears already after 1-2 cycles during the synthesis in cyclic

voltammetry mode, although the polymer growth on the electrode surface continues as

30


the area of cyclic voltammograms grows up (Fig. 1.10). The method of redox catalysis

[111] can be used for lowering the potential of the PANI formation; however, such

complication is, perhaps, unnecessary because of autocatalytic reaction behaviour.

Figure 1.10. Cyclic voltammograms of the PANI electrochemical synthesis. Media –

aqueous solution containing aniline (0.1 М), Nа2SO4 (0.5 М) and Н2SO4, рН 1.0 [114].

Potential sweep rate 50 mV/s. Potential – vs SCE:

1 — first; 2 — second; 3 — tenth cycle

The rate of the electrochemical polymerization reaction is directly proportional

to the aniline concentration and to the concentration of the acid [112]. It has been also

determined that the limitation stage of the process is the aniline oxidation resulted in the

formation of cation radical. The measurement performed in galvanostatic mode also

confirms the existence of induction period [109]. The obtained results again confirm the

above assumption that acid anions also take part in the aniline polymerization process

and they can considerably influence the rate of polymerization.

However, the data of the redox PANI transitions mentioned above do not give

a simple explanation of the mechanism of the aniline oxidation process. Among

different proposed mechanisms of polymerization the most real one seems to be a

mechanism of the chain radical polymerization (scheme 1.10). According to this scheme

after the cation radical formation at an electrode surface, the reaction of cation radical

with monomer molecule and the loss of proton take place. Further oxidation of aniline

monomer and the growth of the polymer chain take place after repeated oxidation of the

dimer and the loss of proton. The confirmation of the fact that the aniline

polymerization process is accompanied with the process of proton exchange was found

31


Scheme 1.10.

а) monomer oxidation:

b) loss of proton:

c) growth of radical:

d) repeated oxidation and proton loss:

e) further chain growth:

in the article [113].

The stability of the cation-radicals is a very important factor for the

electrochemical polymerization process. Cation radicals of a middle stability display a

chemical selectivity toward the chain polymerization [12]. More stable cation radicals

diffuse from the electrode surface that leads to the oligomers and to the PANI

formationin the solution volume. Less stable cation radicals are more reactive.

Therefore, they react with the solvent and with other nucleophilics near the electrode

surface before they had time to diffuse out from the electrode. Thus, the electrochemical

aniline polymerization in the presence of high nucleophilic agents (halogens,

NH

H-e +-

NH

H. . .

+

H

H+

- HN

H

N+N

H

H.. NH2

H +. . ..

++

NH

H - H;

.. NH2

+H

. H

HN

H

N. ..- 2 e

. H

HN

H

N.+ +

-

. H

HN

H

N.+ +NH2+ ..

.N

H

N.

H

NH2. . ..NH2+ ..

Polymer

-2H+

-2e-

32


hydroxides) can lead to a decrease of the polymer yield and molecular weight and, also,

to the formation of soluble products.

As for subsequent stages of the polymerization process, the following

mechanism has been widely spread in the literature [6, 86, 111, 113]. Firstly, PNA

formation takes place. Secondly, pernigraniline further transforms to the final product –

emeraldine. The transition of PNA to emeraldine is a reduction process, in which PNA

plays a role of the electron transfer mediator as it is rather strong oxidizer. According to

this mechanism an anilinium cation formed in the course of the aniline polymerization

(scheme 1.11) [115]. This cation is very reactive and takes part in the process of the

PANI synthesis too. Such mechanism of polymerization received the name of a

“catalytical process” [86].

This mechanism seems to be more probable than that proposed by Sasaki et al

[37] according to which all aniline molecules are oxidized to cation radicals. This cation

radical is formed during the electrochemical oxidation process on the electrode surface

according to the scheme 1.12. Such high reactivity of rather delocalized polymer cation

radical gives rise to doubts.

Proceeding from catalytic mechanism of the aniline polymerization, it was

(1.12)

(1.11)

NH NH+.

n+1

N y + NH3

+NH+.

y + NH2

+.

-e, -H+-PNA E

NH n NH

+.+ H2N

- H+

NH n NH +

NH

H+- H- e-

NH n+1 NH - e

33


established that kinetics of the growth of subsequent polymer layers is influenced by

structure of the formed PANI layer [29, 106].

The obtained results [9] confirm the catalytical mechanism that includes the

stage of the PNA formation. According to these results the induction period during the

polymerization process is observed at the aniline oxidation in 0.5 М aqueous solution of

sulphuric acid on a gold electrode surface in the potentiostatic mode (Fig. 1.11). It can

be supposed that the whole quantity of electricity in this initial period is consulted for

the formation of double electric layer, for the adsorption process and for the formation

of oligomer layer on the electrode surface. Only after the formation of this double

electric layer, i.e. after the induction period, the current increase begins.

Fig. 1.11. Dependency of current density і and charge q on time during the

potentiostatic PANI synthesis [9]. Gold electrode, potential 1 V (vs standard hydrogen

electrode (SHE))

1, 2 — 0.5 М Н2SO4 + 0.01 М aniline; 3 — 0.5 М Н2SO4 without aniline,

I — double layer; II — adsorption; III — oligomer formation;

IV — nucleation, polymer growth

All these mechanisms, as it was already said, do not take into account the

anion, though the morphology of formed PANI depends on it [48, 52, 61, 64]. Granular,

porous structure is characteristic of PANI formed in aqueous solutions of perchloric and

acetic acids, and PANI obtained in sulphuric, hydrochloric or nitric acids has dense

unporous structure

34


1.2.3. Comparison of chemical and electrochemical methods of the

polyaniline synthesis

The chemical way of synthesis is rather simple and cheap in comparison with

electrochemical one. However, the electrochemical method has several advantages over

the chemical polymerization.

Firstly, it is more preferable when the polymer product is intended to be used

as a polymer film electrode, thin layer sensor, or in microtechnology because the

potential or current control is a precondition for the production of high-quality material

and the polymer film that serves as an anode during synthesis is formed at the desirable

spot.

Secondly, the electrochemical way of the PANI synthesis makes it possible to

receive films of different thickness and morphology by a simple change of electrolysis

conditions, electrolyte composition and temperature.

Thirdly, the current yield is near 100% and this permits receiving films of

necessary thickness.

At last, fourthly, the electrochemical polymerization enables us to avoid by-

products of the process implying that PANI obtained in this way is pure, without any

impurities.

Potentio- and galvanostatic modes of the PANI synthesis can be considered to

be close to their final results as the current and potential reach steady value after some

transition period. These methods are rather simple in realization. However, it was noted

that films formed in the cyclic voltammetry mode have a better adhesion to electrode

[48], better optical characteristics, they are more homogeneous and dense [85]. The film

homogeneity decreases with increasing the potential sweep rate [42]. The most

qualitative and homogeneous PANI films are formed with decreasing the aniline

concentration [85].

At the same time, electrochemical method has rather essential disadvantages –

the size of the conducting films formed by this method depends on the electrode

surface, which is not very convenient for obtaining such films on a large scale. Besides,

it is rather difficult to obtain thick, more than 150-300 nm, homogeneous and dense

PANI films.

35


1.3. Mechanism of electrical conductivity in polyaniline

The conductivity of a material depends on its electronic energy level structure.

In a crystalline solid, the degenerate atomic energy levels of atoms or molecules are

combined to form non-degenerate energy band. The width of these bands depends upon

the strength of interaction between atoms or molecules, and the wave functions

describing electrons in these band states extend over the solid. The energy difference

between the highest occupied band (valence band) and the lowest unoccupied band

(conduction band) is called the band gap. The electrical properties of conventional

materials depend on how the bands are filled. Conduction occurs when an electron is

promoted from the valence band to the conduction band, but this can not occur when the

bands are empty or full [116]. If the band gap is small, then thermal excitation can be

enough to give rise to conductivity. That is why it happens in conventional

semiconductors.

The electronic ground state of the conducting polymers is that of an insulator,

with a forbidden energy gap between the valence and conducting bands. The

conductivity of these polymers is transformed through the process of doping [10]. The

term doping is derived by analogy with semiconductor systems. However, in contrast to

semiconductor systems, doping does not refer to the replacement of atoms in the

material’s framework. Doping in the case of a conjugated polymer refers to the

oxidation or reduction of the π-electronic system, p-doping and n-doping, respectively,

and can be effected chemically or electrochemically. To maintain electroneutrality,

doping requires incorporation of a counter-ion. Moreover, the conductivity of a polymer

can be increased by several orders by doping it with oxidative/reductive substituents or

by donor/aceptor radicals. Doping is accomplished by chemical methods of direct

exposure of the conjugated polymer to a charge transfer agent (dopant) in the gas or

solution phase, or by the electrochemical oxidation or reduction.

PANI holds a special position among conducting polymers in that its most

highly conducting form can be reached by two completely different processes – protonic

acid doping and oxidative doping. Protonic acid doping of EB units with, for example,

1M aqueous HCl results in a complete protonation of the imine nitrogen atoms to give

the fully protonated emeraldine hydrochloride salt [17]. Upon protonation of EB to ES,

36


the proton induced spin unpairing mechanism leads to a structural change with one

unpaired spin per repeat unit, but with no change in the number of electrons [94]. The

same doped polymer can be obtained by chemical oxidation (p-doping) of LE base

[117]. This actually involves the oxidation of the σ/π system rather than just the π

system of the polymer as it is usually the case in p-doping [118].

Neutral (non-doped) PANI – leucoemeraldine (Fig. 1.1) – is a dielectric with

the width of band gap 3.9 eV [119]. During oxidation (doping) π-electrons are removed

from the top levels of valence band and the shift of the boundary π-levels (higher than

occupied and lower than valence bands) occurs towards less energies. The gap between

boundary π-levels (forbidden band gap) becomes less (its value is ≤ 2.7 eV [120]) and

the polymer becomes a semiconductor.

The electrical conductivity and other physical properties of doped PANI are

usually explained in the frame of polaron-bipolaron theory [121]. H. Reiss [18]

proposed for the highly conducting state the structure of polaronic lattice which is in

good agreement with theoretical calculations and optical properties of the polymer.

Figure 1.12 provides a schematic representation of the effect of dopants on the

conductivity of materials with reference to the band gap theory.

Polaron energy band

Bipolaron energy band

d

Undoped polymer

Slightly doped polymer

Heavily dopedpolymer

Figure 1.12. The effect of dopants on the conductivity

The charge carriers’ nature (polaron and bipolarons, wh

doping process) is of special interest in the conduction mechanis

point of view, a radical cation (caused by a dopant acting as an ele

partially delocalized over some polymer segment is called a pola

medium around it. If another electron is removed from the pol

happen. If it is removed from a distant section of the chain it f

Incr

easi

ng e

nerg

y

37

Energy level in

conduction ban

Energy level in

valence band

of PANI

ich are formed during

m. From the chemical

ctron acceptor) that is

ron, as it polarises the

ymer, two things can

orms another polaron,


which is independent of the first. If the electron goes out (removing an unpaired

electron) it forms a dication, which is called a bipolaron. The charged fragment can

travel along the polymer chain by rearrangement of double and single bonds. Low

doping levels give rise to a polaron, while high doping levels tend to give rise to a

bipolaron [116].

The presence in the PANI structure of the basic nitrogen atoms, which are

capable to exchange protons with the media, leads to the possibility of injection of large

quantity of charges, i.e. to acid doping (Fig. 1.13).

Figure 1.13. Conductivity-pH dependency

during emeraldine doping [17]

The intensity of an EPR signal is directly proportional to the concentration of

paramagnetic centres and is also connected with the potential according to the Nernst

equation (1.5): signal intensity increases by one order at potential shift byapproximately

59 mV toward anode region at room temperature [122].

During the further PANI oxidation the EPR signal intensity (look Fig. 1.8,

curve 2) passes through the maximum and sharply decreases [17, 39, 122] — according

to the current on cyclic voltammogram (Fig. 1.8, curve 1) [9, 67], the conductivity value

increases as the potential approaches to the emeraldine salt form. With the growth of

polaron concentration on the polymer chain the polaron lattice is formed.

The conductivity mechanism has been widely investigated for solving both

fundamental and practical problems. To explain the mechanism of conductivity on the

basis of carried out measurements the following models have been proposed:

- fluctuation-induced tunneling (FIT) model;

- charge energy limited tunneling (CELT) model;

- variable range hopping (VRH) model.

In FIT model, regions of metallic conductivity separated by insulating barriers

are assumed, the voltage across which shows large thermal fluctuations. This model was

applied successfully to highly conducting polyacetylene and polypyrrole [123]. Also,

38


FIT model was used to the description of the thermal degradation of the electrical

conductivity of the PANI and polypyrrole composite materials containing polymers

with sulphonic and phosphoryl groups [124].

The CELT model, proposed by Sheng et al [125], describes the charge

transport in the system of metallic particles embedded in a dielectric matrix. It is

considered that the doped (protonated) PANI particles are conducting, and dedoped are

nonconducting. Kivelson [126] supposed that a variation of the conductivity of lightly

protonated emeraldine samples with temperature follows a power law.

The mechanism which is often proposed to explain the dc-conductivity in

disordered and amorphous materials is Mott’s VRH model [127]. The mechanism is

based upon the idea that carriers tend to hop more remote distances to sites which are

more advantageous energetically rather than to the neighbouring, but not advantageous

energetically sites. Besides, the VRH model was used to explain the conductive

property of PANI [128]. In the VRH model [128-130] the temperature dependence T of

conductivity σ follows the relation:

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−=

γ

σσ/1

00 exp

TT ,

where T0 is the Mott characteristic temperature and σ0 is the conductivity at T=∝

σ0 = e2⋅υ⋅R2⋅N(E)

and

)(30 ENkT

B ⋅⋅=

αλ ,

where e = 1.602⋅10-19 C is the electronic charge, υ = 1013 Hz is a phonon fre

[126], λ = 18.1 is a dimension constant [128], α is a localization length of the lo

states (cm), kB = 8.616⋅10-5 eV/K is Boltzmann’s constant, N(E) is the de

localized states at the Fermi level (cm-3⋅eV-1), and R given by

R = γ

απ

/1

)(89

⎥⎦

⎤⎢⎣

⎡⋅⋅⋅⋅⋅ ENTkB

is the average hopping distance (cm). γ is determined by the dimension of the

system. For the one-dimensional, two-dimensional and three-dimensional syste

equal to 2, 3 and 4, respectively.

The average hopping activation energy W (eV) can be estimated by the

)

39

(1.13

:

)
(1.14
(1.15)

quency

calized

nsity of

)
(1.16
studied

ms, γ is

known


hopping distance R and the density of states at the Fermi level N(E) by the following

relation:

W = )(4

33 ENR ⋅⋅⋅π

.

Recently, Li et al [129] have investigated the conductive mechanisms

doped and DBSA-doped PANI, which consist of microparticles and micr

respectively. It has been concluded that the conductive mechanism can be consid

be three-dimensional variable range hopping (3D VRH) for PANI-HCl an

dimensional (1D VRH) for DBSA-doped PANI depending on the formed p

structure.

Acid-dopant was also found to determine the hopping barriers. Thus

established from conductivity-temperature measurements that 2-acrylamido-2-m

propanesulphonic acid (AMPSA) produces lower hopping barriers than CSA

This can be due to comparative geometrical structures of AMPSA and CSA. The

is essentially a branched but flexible structure, while the latter is more spherical

likely to be a more rigid molecule. It is assumed [131] that the PANI-AMPSA

can be packed together more closely, giving rise to smaller interchain distanc

hence, lower barriers for carriers to surmount in hopping between chain

explanation was confirmed by X-ray studies [74].

It should be noted that the PANI conductivity may vary not only depen

the acid-dopant used, but also depending on quantities of residual water retaine

polymer. It is known that PANI in the salt form is very hydrophilic (scheme 1.1

In this case it has been assumed that hydrogen bonds are formed between the p

backbone and water molecules and affect the conductivity. The placing of the dr

salt into the environment containing water steam leads to an increase of its cond

by one-two orders [113]. The electronic conduction mechanism of proton exch

based on this phenomenon [132] described below.

In accordance with this mechanism the role of water consists in providi

)

NH2+ ..

+- e- -+ e

+

+

.

+ H2O NH H3O

NHNH

+

or

40

(1.17

of HCl-

ofibers,

ered to

d one-

olymer

, it was

ethyl-1-

[131].

former

, and is

chains

es, and,

s. This

ding on

d in the

8) [77].

olymer

y PANI

uctivity

ange is
(1.18)
ng such


protonation of nitrogen atoms in polymer, at which the electron transfer between amine

and imine units of different chains would not demand the simultaneous proton transfer

and, thus, be easier [77].

1.4. Practical application of polyaniline

Ability to transfer a charge due to the presence of charge carriers in the PANI

structure causes its very important and various applications. Due to the redox process

combined with the intercalation of anions, the PANI chemical, optical and ionic

properties can be attached.

Materials for energy technologies. The electronic conductivity of PANI makes

possible to use it as a cathode in rechargeable batteries. There are many advantages of

these materials, such as the ease of fabrication, processability, low cost and light weight.

Batteries with the PANI electrode have long life, are rechargeable and can produce

current density of up to 50 mA/cm2 and an energy density of 10 Watt·h/kg [133].

Among the batteries, lithium ones are especially important since they show a high

discharging voltage (∼3.0 V).

The redox properties of PANI have conditioned its utilising as a cathode

material in rechargeable Li battery. But the poor charge-discharge capacity of PANI has

hampered its successful application in this respect. This limitation could be overcome to

some extent by incorporating active cathode materials into the PM [134].

Thin film deposition and microstructuring of conducting materials. In

microelectronics PANI may be applied as charge dissipaters for electron beam

lithography. Electron beam lithography is a direct writing method with a very high

resolution in the sub-micron range. The charging of the insulating electron beam resistor

can lead to a deflection of the electron beam and so it results in an image distortion. To

avoid the problem, conducting resistors or layers must be applied. Water-soluble PANI

was introduced by IBM as a discharge solution [135].

Electrooptical and electrochromic devices. The absorbance changes in PANI

make it quite attractive for fabricating electrooptical display devices [88, 94]. PANI

shows all colours as a result of its many protonation and oxidation forms (Fig. 1.1 and

1.2). The electrochromic properties [105, 120, 136] of this polymer can be easily

employed to produce a number of different electrochromic devices, such as display and

41


thermal “smart windows”. The smart windows absorb some of the sunlight, which

changes colour in response to sunlight or temperature changes, thus saving air

conditioning costs.

The use as a membrane. Due to its porosity PANI can be regarded as a

membrane. It could be used for separating gas or liquids. The free standing (on

supporting substrates) chemically prepared PANI films are permeated selectively by

gases [56]. In general, the larger the gas molecule is the lower the permeability through

the film.

Corrosion protection. PANI can be deposited as corrosion protection layer.

Favourite subject of investigations is mild steel, but dental materials are also discussed

[137]. The efficiency and mechanism of corrosion protection are not yet classified. On

iron an anodic protection is discussed [138]. Due to occurring redox processes, thicker

layers of iron oxide are formed and are then stabilized against dissolution and reduction.

An inhibition is also reasonable due to a geometric blocking and a reduction of the

active surface.

Sensors. In order to control air pollution and to detect combustible, toxic or

noxious gases at low levels, efforts are being made towards the development of simple

and inexpensive semiconducting oxide gas sensors. However, such sensors generally

operate efficiently only at 300 oC. In order to overcome this limitation, new materials

are being developed. It is observed that the vacuum deposited thin PANI films exhibit

excellent gas sensing properties [45]. The electrical conductivity, optical absorption and

electrical capacitance of the metal-polymer interface are strongly influenced by the

presence of certain gas molecules. These results have led to the development of gas

sensing elements for different substances, for example for gases like CO, NH3, HCl,

SO2, Cl2 and NO2, for glucose, urea, haemoglobin [45, 46, 139-141]. The thin-film

PANI-based gas sensing elements are inexpensive and operate at room temperatures

with satisfactory selectivity for these gases.

To realise the advantages of PANI having a rare combination of electrical,

electrochemical and physical properties, it is very essential to increase the PANI

processability, environmental and thermal stabilities. From recent studies on the PANI

family of polymers it has been demonstrated that these polymers could be highly

promising for many technological uses because of their chemical versatility, stability

and low cost. But in order to get a material suitable for application in various

technological fields one has to overcome certain limitations such as poor mechanical

42


properties and processability. Several approaches have been made by a lot of researches

to improve these properties. Among them: doping PANI by functionalized acids (such

as CSA, DBSA), using substituted PANI, and preparing composites with conventional

polymers. From our point of view, the last way, i.e. the preparation of conducting

polymer composites is the most promising, as in this case we obtain a material, which

combines mechanical properties of common polymers with the electrical conductivity of

PANI.

Main advantages of such composite systems are their rather low cost and

preserving main physico-mechanical properties of a dielectric matrix as in most cases it

acquires the necessary electrophysical properties already after addition of a small

quantity of the conducting polymer (1-10 wt.%).

1.5. Composite materials based on polyaniline and polyamide

Up to now it has been reported on a lot of composite systems based on

different common polymers such as poly(methyl methacrylate) (PMMA), poly(vinyl

alcohol) (PVA), poly(vinylidene fluoride) (PVDF), etc. [4, 5]. Nowadays, good

conductivity results are received in these systems. However, the composite materials

with the polymers of basic nature (specifically, polyamides) are not sufficiently studied.

But a few studies which can be found in the literature on this subject show quite

promising results [142-147].

It is known that the processing method may significantly determine the

properties of the manufactured composite materials. Well known methods to produce

the PANI containing composites may be essentially reduced to such groups [5]:

- chemical in situ polymerization of aniline in the subsurface layer of a

matrix or in the solution with a matrix polymer;

- dispersion polymerization of aniline in the presence of a matrix polymer in

the disperse or continuous phase of a dispersion;

- electrochemical polymerization of aniline in the matrix covering the anode;

- solution blending soluble matrix polymers and substituted polyanilines;

- dry blending followed by melt processing (mechanical mixing of doped

PANI with thermoplastic polymer, then melded in a hot press or extruder).

Each of these methods has its own advantages and limitations. The choice of

43


the used method in each case will depend on the necessary properties of obtained

composite materials.

Electrochemical polymerization of aniline in the polymer matrix. It is

known that the electrochemical polymerization in the PM will be realized only in

conditions of film swelling in proper conducting media for obtaining ionic conductivity

[148, 149]. The swelling process is realized by two ways:

- by the diffusion in pores in the PM film;

- by the swelling of the polymer film in proper conducting media

which contained aniline.

TGA analyses of potentiostatically synthesized composite film based on PANI

and aliphatic PA, coated platinum electrode [146] have revealed that the so-called

composite has a single phase as it is denoted by a single weight loss pattern. Although

the hydrogen bonding (due to C=O group of PA and N-H group of PANI) was clearly

seen in the solid state FTIR spectra, thermal properties could not explain such a strong

interaction. The DSC curves of the film showed one transition above 340 0С without the

PA transition. While the mechanical mixture of PANI and PA had the glass transition

temperature (Tg) of PA only. Obtained in this way polymer films also had very

reversible behaviour towards NH3 and HCl gases [146].

Chemical aniline polymerization in the subsurface layer of a polymer

matrix. This method allows obtaining such composite materials in which PANI is

located in a very thin subsurface layer of the polymer film. It was established that the

thickness of conducting layer and the conductivity of the composite film depend on the

polymerization conditions (method of synthesis, time of polymerization) [5, 143]. The

method of the chemical aniline polymerization allows obtaining conducting composite

materials with wide conductivity range – from semiconductors to conductivity of pure

PANI [150].

The method is based on the polymer film swelling in aniline [142] or in the

oxidant solution [35]. The swelled film is then put into the solution of oxidant or

monomer, correspondingly. The polymerization process takes place inside the film

surface layer. The PANI formation does not occur in the solution volume. The

properties of the formed composite materials depend on physico-chemical interaction

both of PANI with PA [144-147] and of the PM with aniline [151, 152].

44


Byun et al [143] established that the composite film based on PANI and

polyamide-6 (PA-6) prepared by this method in the presence of different acids-dopants

(HCl, DBSA, TSA, sulphosalicylic acid) consists of three layers. The outer two layers

are conducting composite layers and the inner layer is pristine PA-6. It was found that

despite the low percolation threshold (about 4 wt.%) such films have the value of

conductivity about 3.5⋅10-2 S/cm in the case of HCl. In addition, the existence of

hydrogen bonds between PANI and PA was also established [143]. These bonds can

decrease a little the PANI ability for doping [143]. The physico-chemical interaction

between PANI and the dielectric PA matrix was confirmed by the results of dynamical

mechanical thermal analysis results. Specifically, the deformation of the PA-6 crystal

structure because of the formation of PANI was revealed. The decreasing of the degree

of crystallinity (18.7% in comparison with 21.0% for pure PA) and of the heat of fusion

(49.4 J/g and 55.28 J/g for the composite and PA, respectively) [143] also testify to the existence of the interaction between PANI and the PA matrix.

The size of the acid-dopant anion influenced the behaviour of such surface

composite materials. Neoh et al established [142] that the Cl- counter-ions are easily

removed when the film is immersed in water causing PANI to revert to the base form.

At the same time, the film doped with larger anions (for example, sulphosalicylic acid

anions or polystyrenesulphonic acid anions) can remain in the salt conducting form after

being in water for extended time and under simulated weathering.

It should be noted, that produced by this method composite films owing to

small thickness (∼1-2 µm) [152] of the PANI containing layers can quickly react to

environmental influence, which allows applying them in sensor or optical devices [145].

The resistance of the PA-6/PANI composite films increased in the ammonium

containing environment, but after the exposition to ambient air they returned to the

conducting state. It was found [145] that the films doped by formic acid have better

sensor properties to ammonium gas in combination with high selectivity in comparison

with the films doped by other acids (sulphuric acid, maleinic acid, DBSA).

The level of interaction of the PA-6 and PA-12 films with doped and dedoped

PANI was established with the help of DSC measurements [147]. Electron microscopy

results demonstrated that even though the blends are phase-separated, the degree of the

PANI dispersion in the PA matrix is dependent upon the nature of the counterion of the

doping acid. The confirmation of this fact is the changes of the enthalpy of fusion of

45


obtained films (Table 1.2).

Table 1.2. Enthalpy of fusion (∆Hf ) of the composite PA-6/PANI and

PA-12/PANI films [147]

PANI (vol.%) ∆Hf (W/g), PA-6 ∆Hf (W/g), PA -12

0 75 80

10% PANI-EB 60 101

25% PANI-EB 41 130

10% PANI-MSA 67 88

25% PANI-MSA 42 77

50% PANI-MSA 18 73

10% PANI-CSA 71

25% PANI-CSA 62

50% PANI-CSA 63

10% PANI-DBSA 73 87

25% PANI-DBSA 66 99

50% PANI-DBSA 72 111

It was shown [147] that the conductivity of the composite films PA-6/PANI-

CSA does not depend on the content of the conducting PANI-CSA for contents higher

than 35% (wt/wt). Also, it was established that the rate of increasing conductivity

changes depending on the composition of the film. The value of conductivity for the all

PA/PANI films is increased at increasing PANI concentration up to a definite value. It

was found that the conductivity value reaches ≈10-2 S/cm even at 2% (wt/wt) of PANI-

CSA in the film of PA-6.

It was found that the value of conductivity is very sensitive to the morphology

of composite films. The results of optical microscopy showed [147] that films of PANI-

EB with PA-6 or PA-12 appear phase-separated while the blends containing PANI-CSA

appear homogeneous (and transparent for low PANI-CSA concentration) with no

obvious sign of phase separation. However, the phase-separated morphology of either

PA/PANI-CSA blends is evident in their transmission electron microscopy (TEM)

46


micrographs in which dense network-like structure of PANI-CSA in the host matrix is

clearly seen [147]. A study using small-angle neutron scattering (SANS) [153]

established that the blends with the smaller more polar dopants CSA and MSA behave

similarly and are unlike from either the PANI-DBSA blends or those with PANI-EB.

There is evidence that the simple picture of two pure phases is inadequate for these

materials. With the exception of the PANI-DBSA blend which has a relatively low

scattering contrast, the results indicate that the lower limit of volume fraction for the

application of SANS is a few percent PANI-emeraldine salt in PA-6. X-ray scattering

was used also to demonstrate the presence of the polyamide lamellae and residual peaks

attributable to the pure components [153].

Thus, although such distinctly different sulfonic acids (CSA, DBSA, MSA) do

not induce any pronounced interaction between the rigid PANI and the coil-like PA

chains, they do influence the degree of dispersion of PANI in the host polymer [147].

These factors are very important for the properties and the structure of the PANI

percolation network formed in the composite materials.

Analyzing the above mentioned methods of the PANI-containing composites

formation from structural, physical and chemical positions, one can say that the aniline

matrix polymerization (chemical or electrochemical) allows obtaining the composites

with the PANI percolation network distribution which is to a considerable extent

predetermined by the structure of the initial PM. The aniline matrix polymerization is,

obviously, a suitable way to produce materials with adjusted conductivity properties.

On the other hand, the fact that the structure of the initial PM influences the

properties of the final composite material is, probably, a factor limiting the conductivity

of the composite materials. Besides, the matrix polymerization is mainly suitable for

producing small-sized devices. If it is necessary to avoid these limitations, the aniline

polymerization in dispersion of the common polymer (the so-called dispersion

polymerization) [5] may be considered as an alternative way. Good homogeneity and a

low percolation threshold are characteristic of the composite materials obtained by this

method.

1.6. Research motivation

The results and data discussed above allow us making a conclusion that the

47


formation of composite materials based on PANI and common polymers opens new

possibilities for production of the materials with desired conductivity, mechanical,

thermal and other properties. Although a lot of such composite systems have been well

known, their industrial applications are still very limited, and in the case of the

important PA/PANI composites are unknown. This fact can be explained by a lack of

data on effects of the chemistry and physico-chemistry of the aniline polymerization

process in the presence of the matrix polymer on conductivity, thermostability,

mechanical, structural and other properties of the composite materials.

That’s why the investigation of the aniline polymerization conditions, the

investigation of kinetics and mechanism of this process in the different PA systems

(matrix and dispersion polymerization), physical properties of the formed materials are

necessary for a development of the production technology of the composite materials.

The main goal of this work is to obtain new composite materials of different

types by combining PANI and PA in order to take advantage of unique properties of

both materials. This goal will be reached through:

1. Investigation of the kinetic peculiarities of the chemical and electrochemical

aniline polymerization in the PM;

2. Establishment of the mechanism of the aniline polymerization process in the

PM films;

3. Development of methods of determining the real PANI content in the

composite materials;

4. Carrying out a systematic investigation of the influence of the composition

of the PM/PANI composites on the aniline polymerization process;

5. Studying a synthesis-structure-property relationship for the PA/PANI

composite system.

This study will also give a better understanding of the composite material

structure and provide an insight in the electrical properties of such materials.

48

Chapter 2

EXPERIMENTAL PART


Introduction

In this chapter a brief description of the materials used throughout this work

will be provided. Also general details of the experimental procedures employed for the

electrochemical and chemical preparation of the composite materials described in this

thesis are included, as well as the electrochemical, spectroscopic, dielectric and other

methods of characterization used.

2.1. Materials and reagents

PANI as a conducting filler and PA as the PM have been chosen to be the main

system for studying the formation process and the properties of conducting polymer

materials. Specifically, the influence of the PM structure on the properties of the

obtained composites has been established for the most widely used in industry

polyamides of different structure (PA-6, PA-11, PA-12) and also for the matrix of PVA

and poly(ethylene terephtalate) (PET) (Fig. 2.1).

a) b)

c) d)

O C C

O O

O (CH2)2... ...

e)

Figure 2.1. The elementary chain of the PM:

a) PA-6; b) PA-11; c) PA-12; d) PVA; e) PET

C

O

NH... C

O

...(CH2)5 C

O

NH... C

O

...(CH2)10

C

O

NH... (CH2)11 C

O

...OH

CHCH2 CH2 CH...

OH

...

49


The characterization of the polymers which were used as the PM is given in

Table 2.1. The reagents which were used in the course of the work are listed in Table

2.2. All solutions were prepared on distilled water.

Table 2.1. Characterizations of the used polymer materials

Thickness of the film or

particle size of the powder

Kind of polymer

Polymer form Thickness,

µm

Particle size,

µm

Producer

PA-6 film 25 Good Fellow

PA-11 BESHVO film 50 Arkema

PA-11 Rilsan B powder 14 - 42 Arkema

PA-12 film 50 Arkema

PA-12 Orgasol powder 5 Arkema

PA-12 powder 14 - 243 Arkema

PVA powder 15 - 30 Ukraine

PET film 50 Ukraine

Aniline purification procedure. Prior to use, aniline was distilled under

reduced pressure in the presence of zinc powder being a reducing agent. Such additional

purification permitted to remove and to prevent the formation of aniliniquinones and

azophenines [154], which coloured aniline solution in bright red colour. Purified aniline

and all aniline-contained solutions were stored at +3 - +5 0С in the darkness and in the

argon atmosphere for further prevention of the coloured substances formation.

Preparation of chlorine water solution. The water solution of chlorine in

accordance with the technique of [152] was used as one of oxidation solutions for the

chemical aniline polymerization in the PM. The process of saturation of pre-cooled

distilled water by chlorine gas was carried out by a dry Cl2 blowing through distilled

water for 2 hours at +20 0С till complete saturation of the solution with chlorine. The

solubility of chlorine in water is 7.5 g/l under these conditions [155].

It is known that Cl2, when dissolved in water, is hydrolyzed and the formation

of weak acid HClO and strong acid HCl takes place. The rate of this reaction is so high

50


Table 2.2. Characterizations of the used reagents

Name of reagent

Formula

Grade of purification,

producer

Comments

Aniline

C6H5NH2

p.a., Aldrich

After additional

purification, look

above

Hydrochloric acid HCl analytical grade, Ukraine

Sulphuric acid H2SO4 analytical grade, Ukraine

TSA C7H10O4 98% monohydrate,

Aldrich

DBSA C18H31O3S mixture of isomers

С10 - С13, Acros

CSA C10H16O4S p.a., Acros

Acetone CH3COCH3 p.a., Ukraine

n-Hexane С6Н14 p.a., Ukraine

Formic acid HСОOH analytical grade, 99.9%,

Acros

APS (NH4)2S2O8 p.a., Ukraine

NMP C5H9NO Aldrich

Potassium chloride KCl p.a., Ukraine Crystallized twice

from distilled water

Water solution of

chlorine

Cl2 + H2O Preparation look

above

that the equilibrium is established immediately [155]. That’s why concentrations of Cl2

were determined by pH of the obtained solutions [156]. After the saturating process had

been finished, the obtained chlorine water solution was put into glass ampoules, which

were previously carefully washed by warm alkaline solution (60 0С) and distilled water

and then dried. These ampoules after filling by water chlorine solution were soldered

and stored in the dark.

51


2.2. Preparation of the composite materials

2.2.1. Synthesis of polyaniline

The synthesis of pure PANI was performed in order to compare its spectral,

electrical, dielectric and thermal properties with the properties of the composite

materials.

Chemical polymerization. The emeraldine salt form of PANI was

synthesized chemically by the oxidative polymerization. According to [157], aniline (8

ml) was dissolved in 80 ml of 1.5 M HCl aqueous solution at 20 0C and then stirred

during 15 minutes to form an anilinium salt. Another solution containing 24.05 g of

APS in 50 ml of distilled water was prepared. It was then added to the aniline solution.

After the addition of the APS solution, the reaction mixture was stirred for further 15

hours. The precipitated PANI-HCl powder was collected under vacuum filtration and

washed with the distilled water to remove any impurities. The obtained powder was

dried under dynamic vacuum to a stable weight.

The emeraldine base powder was prepared by the alkaline dedoping of the

PANI-HCl emeraldine salt. This was accomplished by stirring the emeraldine salt in a

5% NH4OH aqueous solution for 10 hours. The powder was then washed and dried

under vacuum for 20 hours and stored in a refrigerator for later use.

Thus obtained the dedoped PANI powder was redoped by stirring for 24 hours

in 1M water solution of appropriate acid, producing PANI-acid complex. Finally, the

doped powders were dried under dynamic vacuum at 70 0C for 10 hours till a constant

weight.

Electrochemical polymerization. The electrochemical PANI formation on

the working electrode surface was carried out in 1M HCl water solution, containing

0.5M aniline. This solution was prepared by the dissolution of calculated aniline weight

in the necessary volume of the acid solution right before experiments.

2.2.2. The formation and pre-treatment of the polymer matrix

The PM in both film and powder forms were used for determining the

influence of the PM structure on composite materials properties.

The film samples were of two types:

52


- commercial and industrial PA films (kindly donated by Arkema,

Serquigny, France and purchased from Good Fellow (PA-6));

- films which were made in laboratory:

- by casting method from the solution of the PM;

- by the compression-molding method from the powder.

Formation of films from the PM solution by casting. This method was

used for studying the process of the electrochemical aniline polymerization in the PM.

The solutions of PA and PVA were prepared. The solution of PVA was prepared by

dissolving 1 g of the PVA powder in 80 ml of hot water (65-70 0С) at continuous

stirring. Similarly, the PA solution was prepared by dissolving 1 g of PA-12 in 100 ml

of concentrated and warmed-up (40-50 0С) formic acid. Then, a certain volume of the

solution (0.015 ml) was deposited by syringe on the electrode surface for the film

formation followed by drying for 4 hours at room temperature.

It should be noted that PVA is known to be a water soluble polymer, so it was

necessary to convert it into a water-insoluble state in order to prevent its dissolution in

water solutions during the investigation of the electrochemical aniline polymerization in

the PM. It is known [158] that PVA loses its solubility during heating at temperatures

≈100-120 0С due to the cross-linking of the PVA macromolecules by the reaction of

intermolecular etherification (2.1).

The preliminary investigation of the character of weight mass changes du

heating at these temperatures showed that the complete water removal from the f

takes place in 2 hours. So, this time period was used for the PVA film formatio

further investigation.

Film formation by the compression-molding technique. The formatio

films by this method was performed by means of Specac device, which consists

hydraulic press and a digital temperature controller. The polymer powder of comp

material (∼0.2 g) was put in a rectangular form (S = 20.25 cm2) and then compress

(OHOH

CHCH2 CH2 CH ......- H2O

OH

CHCH2 CH2 CH ......

CHCH2 CH2 CH .....

OH

O

53

2.1)

ring

ilms

n in

n of

of a

osite

ed at

.


controlled temperatures (195, 220 and 240 0С) under the load of 3 t during 1 min. The

thickness of the obtained film was ≈100 µm. The measurements of thickness were

performed with a digital micrometer.

2.2.3. Formation of the surface conductive composites

In order to obtain the surface layered conducting polymer composite, the PM

film was immersed into freshly distilled aniline at 20±2 0С in argon atmosphere. The

gravimetric measurements of weight changes of the PM during swelling process were

carried out by analytical balance ExplorerТМ (“OHAUS”, Switzerland). The previously

weighted film samples were placed in the aniline solution to be swelled during definite

time period. Then the films were taken from the solution, washed by n-hexane to

remove the surplus of aniline from the film surface and dried between two sheets of a

filter paper. After one minute exposition at ambient air the film weight was measured.

After this procedure the film was again immersed in the aniline solution, hold there

during a definite period of time depending on the desired aniline content. Then, all

operations described above were repeated.

The swelling degree α [159] was calculated taking into account the changes in

film weight:

%,100)(

0

0 ⋅−

=m

mmα (2.1)

where m0 and m are the weight of the primary and the swelled films, respectively.

The swelling process in the PM was performed till the achievement of the

definite wt.% of aniline in the film depending on research goals.

Swelled polymer films were then subjected to the chemical aniline

polymerization in different oxidant solutions – water chlorine solutions and solutions of

different concentrations of (NH4)2S2O8 in 1М water solutions of HCl or H2SO4. The

aniline polymerization process was controlled by optical UV-Vis measurements.

In order to obtain one-side surface composites a home-built cell was used (Fig.

2.2). The cell construction permits to block the other side of the PM from the oxidant

solution. The swelled polymer film was placed in the cell and the cell was filled with

the solution of oxidant. After the completion of the process the green transparent film

was taken from the oxidant solution, washed by distilled water and placed in Soxlet

apparatus for 6 h to extract with n-hexane the aniline livings and by-products. This

54


Figure 2.2. Home-built cell used for the obtaining of the one-side surface composite

films

55


procedure was followed by drying the film under dynamic vacuum up to a stable

weight.

In order to convert PANI formed in the PM into the non-conducting EB state

the film was treated with the aqueous solution of 5% NH4OH for 24 hours.

2.2.4. Formation of the bulk conductive composites

The bulk conductive composite materials were prepared on the basis of the PA

powders of different dispersion and structure (PA-11, PA-12). Inorganic (HCl and

H2SO4) and organic (TSA, DBSA and CSA) acids were used as acid-dopants. As TSA

was in a solid state, the concentration of the obtained acid solution was estimated by the

reverse titration by 0.1N solution of sodium hydroxide in the presence of methylorange

indicator.

In order to prepare a serie of the composite samples, different volumes of

aniline monomer are reacted with a constant weight of the PA powder.

For the preparation of anilinium salt the estimated amount of aniline was

dissolved in the necessary volume of proper acid solution. Thus formed solution of the

anilinium salt was vigorously stirred by magnetic stirrer for 0.5-2 hours. Then a definite

amount of the PA powder was added to this solution and thus obtained dispersion was

continuously stirred during one hour till the formation of homogeneous dispersion

mixture. After this, the solution of oxidant (water solution of APS) was added to the

dispersion.

The preliminary investigation has shown that the optimal condition of the

aniline polymerization in water dispersion of the PA powder for obtaining conducting

composite materials is the maintaining of the following molar ratios:

5.11

=acid

aniline ; 25.11

=oxidantaniline

Depending on the acid nature, in 0.1-1 hour the colouring of solution into dark

green colour took place, which is in the agreement with literature [104] and is

responsible for the formation of PANI in the emeraldine salt state. The obtained

dispersion was left for further 15 hours to be sure to complete the aniline oxidation

process. For that time the mixture was continuously stirred.

After the completion of the process the obtained dispersion of dark green

colour was flooded with the 10-fold surplus of distilled water and stirred by magnetic

stirrer for 15 minutes. This was made for the removal reagent livings and soluble by-

56


products. Then the precipitated PA/PANI powder was filtered through a Buchner filter

funnel. After the filtration the product slurry of the composite material was placed into

the flask and dried under vacuum at 70-80 0С to a constant weight.

In the case of TSA using, the slurry of the composite material after filtration

was divided into two parts. One part was placed into the flask with 15 ml of 0.2 М

solution of TSA for 24 hours. Such additional treatment in TSA solution was necessary

because of TSA good solubility in water. Due to this, partial removal of TSA from the

composite powder can occur during the water washing. After the additional treatment

by acid solution the powder was filtered and was washed on filter by 30 ml of distilled

water to remove TSA surplus. The other part of composite powder was left without

additional treatment. Both parts of each sample were placed into round-bottomed flasks

and dried under vacuum at 70-80 0С to a constant weight.

A part of resulting powder was being neutralized with 5% NH4OH aqueous

solution for 24 hours. The precipitated composite was washed repeatedly with distilled

water till the filtrate became free of alkali (pH 7) and then was dried under vacuum for

48 hours. Thus obtained dedoped composite material was redoped by stirring for 24

hours in 1M water solution of appropriate acid, producing doped PANI complex.

Finally, the polymer doped powders were dried under dynamic vacuum at 70 0C for 10

hours till a constant weight.

Thus obtained composite powders were used for further investigation both in

the powder and film forms made by the compression-molding technique (look section

2.2.2).

2.3. The main investigation methods and techniques

The complex of physico-chemical methods of investigation (such as the cyclic

voltammetry, UV-Vis spectroscopy, dielectric relaxation spectroscopy, Raman

spectrometry, thermogravimetrical analysis, conductivity measurements, pH-potential-

temperature and swelling degree measurements, microscopy studies) were carried out

for studying the influence of the synthesis conditions, the structure of the PM, the nature

of electrode material and acid-dopant on the process of the formation of the composites

based on PANI, for investigating the effect of a composition of the materials on their

physicochemical properties.

57


2.3.1. Method of determination of the real polyaniline content in the

composites

The possibility to determine the real content of formed PANI plays a very

important role for the kinetic investigation of the aniline polymerization. The most close

to this task is the method of determination of the PANI oxidation state in NMP by

measuring the consecutive UV-Vis absorption spectra [160]. It should be noted that

authors examined only pure PANI, but not PANI obtained in the composite materials.

And, besides, it needs some modifications allowing the use of unknown concentrations

of PANI in the presence of the PM. So, the necessity of a new method of the PANI

content determination arose in the course of the study.

Our method is based on determining the relationship between the optical

absorption density D and the PANI concentration in the polymer composites. It was

found that it is possible to use PANI in both EB and LE oxidation states. For this

purpose it is necessary to create the calibration plots for PANI in these oxidation states.

These states were chosen due to the independence of UV-Vis spectra profile of

these PANI states of the used acid-dopant unlike from the emeraldine salt form [6].

And, besides, it is known that UV-Vis spectrum of LE is characterized by one well-

defined absorption peak [6]. However, it is rather complicated to convert PANI formed

in thin subsurface layer of the PM to the LE oxidation state. Of this point the EB state

was chosen for the surface composite films and LE – for the bulk composite materials.

The synthesized EB (look section 2.2.1) was dissolved in the solution of NMP

at continuous stirring for 5 hours. This process took place in a flask with NMP seal to

prevent the evaporation of the solvent.

A series of solutions with different PANI concentration were prepared by the

subsequent dilution of the primary PANI solution. The received UV-Vis absorption

spectra of these solutions are characterized by two bands – one at λ = 331 nm (assigned

to π-π* transition of benzene rings) and the other one at λ = 647 nm (absorption of

quinonediimine groups). Two calibration plots of optical absorption density and the

PANI concentration in solution were depicted on the basis of the received results (Fig.

2.3a). The reproducibility of obtained results was checked by conducting of 5 parallel

measurements. The possibility of using the obtained calibration curve for the PANI

content determination in the PA matrix was preliminary postulated on the basis that

both the PM (aliphatic polyamide) and solvent (cyclic amide) had a similar nature. And,

58


Figure 2.3. Dependencies of absorption density on the PANI concentrations in NMP

solution at fixed wavelengths:

а – PANI in emeraldine base state;

b – PANI in leucoemeraldine state

PANI concentration in NMP, %0,000 0,005 0,010 0,015 0,020 0,025 0,030

Abs

orba

nce

0,0

0,5

1,0

1,5

2,0

2,5

331 nm647 nm

PANI concentration in NMP, %0,000 0,002 0,004 0,006 0,008 0,010

Abs

orba

nce

0,0

0,2

0,4

0,6

0,8

1,0

phenylhydrazine, 343 nmascorbic acid, 343 nm

(b)

(a)

59


besides, PANI-base in the solid PA matrix and in the liquid NMP has similar UV-Vis

spectra, i.e. the presence of PA does not influence on the PANI bands position. This

supposes an approximate equality of the PANI extinction coefficients in the both media.

For the determination of the PANI content in the surface composite films the

obtained doped composite material was converted into the dedoped state by the

interaction with the 5% aqueous solution of NH4OH during 20 hours. The comparison

of the optical density of this film at 331 and 647 nm with the calibration curve (Fig.

2.3a) give us the real PANI content in composite.

However, during the dissolution of the bulk PA/PANI composite powder in

NMP we were confronted with difficulties. As it was discovered, although pure PANI

and pure matrix polymer are soluble in NMP, the composite with PANI in the EB form

is insoluble in NMP. That’s why it was necessary to convert PANI into the form which

would be rather well soluble in NMP. It is appeared that PANI in the oxidation state of

LE has a good solubility in NMP and, what is very convenient for the analysis, - PANI

in this oxidation state has only one absorption band at λ = 343 nm (assigned to π-π*

transition of benzene rings) (Fig. 2.3b). The PANI conversion into LE oxidation state

can be made either by ascorbic acid [160] or by phenyl hydrazine [161]. After adding

the solution of the reducing agent the obtained solution was stirred for 8 hours, filtered

from insoluble PA and the UV-Vis spectrum was measured. It was found that both

reducing agents (ascorbic acid and phenyl hydrazine) lead to similar results (Fig. 2.3b).

2.3.2. Electrochemical investigations

The study of the electrochemical aniline polymerization was carried out with

the help of a PI-50-1 model potentiostat/galvanostat, arranged with PR-8 model

programmer (Belorussia) in a three-electrode electrochemical cell with unseparate

compartments (Fig. 2.4). The four-cable circuit board connection of the electrochemical

cell was used for increasing the accuracy of measurements. In this circuit board the

working electrode is connected to the potentiostat by a separate wire.

A variety of electrode materials was used as working electrodes. The

investigation of the aniline electrochemical polymerization was carried out on platinum

electrodes and on transparent glass electrode with conducting SnO2 layer deposited on

one side (transparent SnO2-glass electrode).

The platinum electrodes were chosen due to their stability at anode potentials.

The choice of the SnO2-glass electrode is connected not only with its stability,

60


Figure 2.4. Three-electrode electrochemical cell used for the electrochemical

polymerization

Platinumcounter electrode

Saturated calomel reference electrode

Working electrode

61


but also with the possibility of measuring UV-Vis spectra of the PANI layer formed

directly on the electrode surface.

Two types of platinum electrodes were used: a platinum wire (∅ 0.5 mm,

Svisible = 1.96⋅10-3 cm2) and a platinum plate (S = 3.38 cm2). Also, transparent SnO2-

glass electrode (S = 1.04 cm2) was used. The surface of this glass electrode was limited

by a transparent inert sticky film.

To obtain reproducible results the careful electrode surface treatment was

realized before the experiments. The surface of platinum electrodes was treated in

accordance with the most common method [29, 30, 37]: it was polished on fine-grained

emery paper (Carbimet, Grid 600, USA), then degreased by sodium bicarbonate,

washed carefully by distilled water and dried by the filter paper. The surface of SnO2-

glass electrode was washed by acetone and distilled water and dried by the filter paper.

Counter electrode (cathode) was also a platinum plate, a surface of which is by

two orders higher than the surface of working electrode. Calomel electrode which is

filled by the saturated aqueous solution of potassium chloride KCl (SCE) was used as

the reference electrode. Its equilibrium potential is equal to Е = 0.244 V versus SHE

[162]. All potentials reported here are versus SCE.

The PANI electrochemical synthesis was carried out in galvanostatic,

potentiostatic and cyclic voltammetry modes.

A cyclic voltammetry was used to characterize electrochemical properties of

PANI. The cyclic voltammograms were registered at the potential scan rates νscan = 5,

10, 20, 50, 100 mV/s in the potential range from -0.2 V to 0.8 V. The potential range

was chosen so that it contained the potential of the aniline oxidation (+0.6 V) [163] and,

respectively, the potential of the polymerization process initiation, as well as all charge-

discharge processes of formed PANI [163]. The limitation of the working potential

range by 0.2 V is caused by the fact that the potentials of obtaining PANI are reached.

The upper potential limit (0.8 V) is caused by the fact that during the potential cycling

to more positive values the irreversible degradation of formed PANI takes place because

of the PANI overoxidation [35].

The cyclic voltamogramms were registered by a two-dimensional X-Y recorder

LKD4-003 (Belorussia).

62


The galvanostatic polymerization of aniline was accomplished by applying

current density in the range of 0.001-10 mA/сm2 (for the different electrodes) to the

working electrode for a pre-defined period of time.

In the potentiostatic mode the PANI films were prepared by applying potentials

of 0.6; 0.7; 0.75 and 0.8 V to the working electrode for a certain period of time.

Prior to measurements the electrochemical cell was washed carefully by

distilled water. The reference electrode was connected with the electrochemical cell by

two intermediate electrochemical keys (salt bridge). One of the keys was filled with the

saturated solution of KCl and was placed into the other one, which was filled with the

solution of the appropriate acid. In its turn, the second key was placed into the

electrochemical cell filled with the working solution. After filling the electrochemical

cell and both keys the prepared working electrode was placed into the cell. To increase

the accuracy of measurements and to support the working electrode potential the IR

compensation scheme was used. With the help of this scheme the potentiostat provided

the compensation of the voltage drop at the ohmic resistance of the solution layer in the

electrochemical cell between the end of the electrochemical key and the working

electrode surface.

The voltammetry curves began to be measured after the exposure of the

working electrode in the solution till the approximately constant stationary potential of

the working electrode (Еst ≈0.4 V). After the polymerization was completed, the

resulting films were washed with the distilled water before further use.

All measurements were carried out at ambient temperature (∼20 0С).

2.3.3. Electrical conductivity measurements

The surface conductivity (σ) of the obtained polymer PA/PANI composites

was measured by standard two- or four-electrode methods at ambient temperature (∼20 0С). The thickness of pellets and films was measured with a digital micrometer. Also, it

should be noticed that for the surface conducting composite film the thickness of the

conducting subsurface layer estimated by Raman spectrometry (see section 2.3.4) was

used.

Measurements of sample conductivity less than 10-5 S/cm were carried out with

a digital combined device Щ402-М1 (Ukraine) by measuring resistance by a two-

electrode method. The conductivity value is then calculated using the following

equation:

63


hbRl⋅⋅

=σ , (2.3)

where l is the distance between electrodes (1 cm), cm; R is the measured sample

resistance, Оhm; b is the width of the sample, cm; h is the sample thickness, cm.

The electrical conductivity higher than 10-5 S/cm was measured using a four-

electrode method. Under this method, the film was laid across four parallel electrodes as

shown in Fig. 2.5. A constant current was passed between the two outer electrodes

while the potential drop across the inner electrodes was measured. Measurements were

performed with the help of the universal voltmeter В7-21А (Russia), universal

voltmeter-electrometer В7-30 (Russia) and external current source – accumulator

battery. The conductivity value is then calculated using the following equation:

hbUlI⋅⋅

⋅=σ , (2.4)

where I is the applied current, А; l is the distance between the two central electrodes (1

cm), cm; U – is the measured potential drop, V; b is the width of the sample, cm; h is

the thickness of the sample, cm.

The conductivity measurements were carried out on direct current, and all

electrodes were placed on one side of the sample surface.

Figure 2.5. Configuration used for the measurement of electrical conductivity of

conducting samples using a four-electrode method

I applied

U measured

Sample

Electrodes

vvv

64


The resistance measurements were averaged out of the results of (i) three times

for each film or pellet, (ii) two films or pellets for each sample.

The four-electrode technique was also used to investigate the surface resistance

R of the conducting composite films as a function of the temperature. Four cooper wires

were attached in parallel on the sample surface by conducting silver paint for better

electrical contact. In this case the resistance values were measured using a Yokogawa

7563 electrometer. The temperature dependence of resistance was measured by raising

the temperature at the rate of 1 0C/min in the cryostat equipped with temperature-

controlled heater. The temperature control was realized with the help of Novocontrol

broadband dielectric spectrometer (description of the device look in section 2.3.5). Also

other kind of equipment (Agilent E3634A) was used in order to measure the

temperature dependence of resistance. In this case the temperature was increased with

the rate of 0.2 0C/min. The performed measurements with both type of equipment gave

good reproducibility.

2.3.4. Spectroscopy investigation

UV-visible spectroscopy. The choice of this method is motivated by two

reasons. Firstly, this method enables us to estimate the structural and quantitative

composition of the obtained PANI films. And, secondly, this method also permits to

watch in situ the kinetics of the PANI formation in the PM.

The spectral investigation was performed by a spectrometer Specord М40

(DDR) in the wavelength range of 200 and 900 nm in quartz cuvettes with a path length

of 0.1 and 1 cm.

The kinetic control of the system changes during the chemical aniline

polymerization in the PM was carried out directly in a spectrometric quartz cuvette

(volume: 3 ml, path length: 1 cm). Prior to measurements the PM swelled in aniline was

placed in the cuvette, which was filled with the oxidant solution. The time at which the

swelled PM is put in an oxidant aqueous solution was noted as a starting time of the

polymerization. The electronic absorption spectra were recorded in definite time periods

after the polymerization process began. In some cases, to simplify and to specify the

kinetics control of the aniline polymerization process in the PM, the optical density D of

the system was measured periodically at fixed wavelengths corresponding to the PANI

bands.

65


The spectra of electrochemically obtained PANI on the transparent SnO2-glass

electrode were measured also using this spectometer. PANI covered SnO2-glass

electrode was installed in the spectrometer in the place of quartz cuvette.

The spectra of both chemically and electrochemically formed PANI were

recorded by means of a special software (Spectra). In order to obtain the spectra of pure

PANI, the background spectra of a pure SnO2-glass electrode or a virgin polymer film

were subtracted from the recorded spectra.

Resonance Raman spectrometry. The choice of this method is motivated by

the possibility to characterize and to determine the PANI structure [164-167]. It is well

known that this method enables to investigate objects very small in size (down to a few

µm2) without any damage of the investigated material. This method also allows

obtaining the information about quantity of PANI and its oxidation state in the

composites by analyzing the evolution of the Raman bands intensity.

The experiments were carried out by a multichannel Jobin-Yvon T64000

spectrometer connected to a liquid N2 cooled ССD detector and equipped with a

confocal microscope. The Raman spectra of the powder and polymer films were

recorded with the green excitation laser line (λ = 514.5 nm) of an Argon-Krypton laser

by a special software Labspec3t2. This line was chosen since it allows obtaining

simultaneously both Raman bands of the phenyl units of PANI which are enhanced in

the blue range, and Raman bands of the quinone units which are enhanced in the red

range [167]. Thus, this excitation line allows getting general view of different polymer

forms of PANI. In order to investigate the vibrational properties of different polymer

forms of PANI, the Raman measurements were focused mainly within 1100-1700 cm-1

region.

In order to avoid any local degradation and perturbation of the polymer

samples, the laser beam power was limited to 0.04 mW on a sample surface and the

spectra were collected with an integration time of 300 s. The laser beam was focused on

the sample in a ∼1 µm spot size by an ×100 objective and the spectral resolution was

approximately 1.5 µm.

Taking into account the advantage of the confocal system, Raman spectrometry

was used also to determine the thickness of the conducting layer of the polymer

composite film. For this purpose the Raman spectra were recorded through the thickness

of the composite film using a PI motor stage allowing precise step displacement along

66


the Z axis of the sample below the objective. The first spectrum was obtained by means

of focusing the laser beam on the surface of the film. The following spectra were

collected by focusing the beam inside the composite film with the submicrometric step

between two spectra. The process continued until the end of the conducting layer. Then

the bands of the obtained spectra were compared with the characteristic bands of the

doped PANI spectrum.

The thickness of the conducting layer was found by analyzing the evolution of

the Raman band intensity along the Z displacement. The obtained value of the

conducting layer thickness was used in further calculations of the PANI yield and in

determining kinetic peculiarities of the PANI formation in the polymer films.

2.3.5. Dielectric relaxation spectroscopy (DRS)

It is known that the dielectric behaviour of the composite materials and their

conductivity vary with the structure of polymer chain, doping level, the nature of acid-

dopant, etc. [168-173]. They are also greatly affected by the film-formation method and

the selection of the dopant and solvent [171, 172]. The method of DRS allows

investigating the conductivity mechanism on the basis of measurements of dielectric

parameters of the conducting polymers [168, 169].

The application of an electric field provokes the fluctuations of the electrical

polarization and this leads to the electrical displacement. Such displacement depends on

the material polarization. Dielectric polarization arises due to the existence of atomic

and molecular forces, and appears whenever charges in a material are somewhat

displaced with respect to one another under the influence of an electric field. One can

distinguish several types of polarization [174], which will depend on the applied

frequency and temperature:

- electronic polarization – this effect involves distortion of the center of charge

symmetry of the basic atom. Under the influence of applied field, the nucleus of an

atom and the negative charge center of the electron shift, creating a dipole. This

polarization happens at very high frequencies or at very low temperatures;

- atomic polarization happens at IR frequencies and corresponds to the atoms

displacement in the case of the molecules deformations.

The real part of the dielectric complex permittivity ε’ after this two kinds of the

polarization is noticed as ε∞ - it is minimal value of ε’ observed for the studied

frequency region.

67


- dipoles orientation polarization - this is a phenomenon involving rotation of

permanent dipoles under an applied field;

- the electrode polarization lies in the charge accumulation near the electrodes

and resulted from the ionic conductivity. These charges create an electric field with an

opposite direction to the applied electric field. Such polarization appeared at very low

frequencies;

- Maxwell-Wagner-Sillars (MWS) effect – if the material consists of different

phases, which possess different values of permittivity and conductivity, so in this case

the charges can accumulate at the limits of these phases. Hence, the relaxation time of

the MWS relaxation will depend on the conductivity and permittivity values of the

different components.

Dielectric permittivity measurements were performed using Novocontrol

broadband dielectric spectrometer (Novocontrol GmbH, Germany) (Fig. 2.6) in wide

frequency (0.1Hz to 1GHz) and temperature (173 to 423K) ranges. The spectrometer

consists in the following parts:

- impedance analyzer SOLARTRON SI 12690;

- impedance analyzer HP 4291A;

- dielectric converter NOVOCONTROL;

- temperature controller NOVOCONTROL Quatro Cryosystem;

- cryostat;

- sample cell.

To cover the frequency domain from 0.1 Hz to 1 GHz, two experimental

configurations are employed. From 0.1 Hz up to 10 MHz a SOLARTON SI 1260

analyzer combined with a broadband dielectric converter allows impedance and

dielectric measurement. The high frequency impedance analyzer is used with a

precision coaxial line (Fig. 2.7) [175].

For both configurations, the sample cell consists of two round golden parallel

electrodes filled with the material to form a capacitor. In the case of the high frequency

range the sample capacitor is used as a termination of a golden coaxial line and the

impedance is then calculated from the complex reflection factor at the end of the line.

The polymer film or pellet was placed between two electrodes as sketched in Fig. 2.8.

In the case of the bulk composite materials (Fig. 2.8a) the polymer powders were

compressed into discs of 10 mm in diameter and with thickness of about 1 mm under a

68


Figure 2.6. Schematic representation of the dielectric spectrometer

69


Figure 2.7. Schematic presentation of a high-frequency sample cell [175]

Sample

Electrodes

(a) (b)

Figure 2.8. Sample cell configuration for dielectric measurements in the case of (a) bulk

and (b) surface layered composites

pressure of 3 t at room temperature. The surface containing PANI composite films were

facing each other as indicated in Fig. 2.8b.

The sample temperature was varied by using a gas stream of nitrogen and the

measured temperature close to the sample is controlled within the accuracy of ±0.2 K.

The measurements determined the impedance of the sample capacitor Z

s

s

IU

iZZZ =+= "' , (2.5)

which is connected with the dielectric function of the sample by the equation:

02"'

CfZiisπ

εεε −=−= , (2.6)

where f denotes frequency and C0 is the vacuum capacity of the sample capacitor.

The method of dielectric analysis was chosen to calculate dielectric parameters

of the process (dielectric constant, imaginary and real permittivity, conductivity,

relaxation time, etc.). The values of imaginary and real parts of dielectric permittivity

70


ε*(ω) = ε′ - іε″ were analyzed according to the empirical Havriliak-Negami (HN)

function [174]:

( )( )( )βαωτ

εωεσ

εωεi

i n+

∆+−= ∞

∗

1)(

0

0 , (2.7)

where ω = 2πf; ε0 denotes the vacuum permittivity; σ0 is the dc-conductivity; n is the

exponent factor, in most cases equal to 1; ∆ε is the difference between low and high

frequency limits of ε′ over the relaxation to which the HN function is applied and it is

also proportional to the area below the curve of the ε′′ relaxation peak; ε∞ denotes the

high frequency limit of the permittivity and is the unrelaxed value of permittivity; α and

β are shape parameters; τ is the relaxation time. Processing the experimental dielectric

spectra ε∗(ω) was made by means of WinFit 2.4 (1996) software of Novocontrol GmbH

(Germany).

The characteristic relaxation times τ were taken at the position of the maximum

of dielectric loss for each relaxation process and they were fitted according to the

equation (2.7). The temperature dependences of these relaxation times were analyzed

using an Arrhenius equation (2.8) and activation energies of relaxation processes were

obtained.

)exp(0 TkE

B

aττ = , (2.8)

where τ0 is the relaxation time at very high temperature; Еa is the activation energy; kB

is the Boltzmann’s constant ( 8.616⋅10-5 eV/K).

2.3.6. Thermal analysis and differential scanning calorimetric

measurements

TGA was carried out with a thermogravimetric analyzer MOM (Hungary). The

thermogram of the polymer sample was recorded in the range of temperatures: from

room temperature up to 1000 0C with a heating rate of 10 0C/min in air media. The

weight of the sample was ∼120 mg.

DSC measurements are used in order to obtain a qualitative comparison of the

crystallinity extent of the PM. DSC thermograms of the polymer films were obtained

from heating the samples from -50 0C to 300 0C under N2 purging at 20 0C/min by a

TA-Instruments differential scanning calorimeter. The weight of the sample was ∼2 mg.

71


The ratio of the fusion enthalpy of the semicrystalline samples ∆Hc over the

fusion enthalpy of the pure (100%) crystalline polymer ∆H° gives the degree of the

crystallinity, Xc, of each polymer [147]:

%100⋅∆∆

=°H

HX c

c (2.9)

2.3.7. Mechanical analysis

The mechanical properties of the polymer composite films were measured on

the film strips of 10×44 mm in dimension. The measurement of their tensile strength

was performed by using a tensile-testing machine at room temperature (∼20 0C) using a

cross-head speed of 50 mm/min. For each tensile strength reported, at least three sample

measurements were averaged.

2.3.8. Optical and atomic force microscopies

The method of the optical microscopy was used in order to obtain structural

information about the composite PM/PANI samples. The electron micrographs of the

composite films were performed using a Leica DMRX polarizing microscope.

The atomic force microscopy (AFM) (electrical and topographical images) of

the composite films were realized in the Laboratoire de Génie Electrique de Paris, UMR

CNRS 8507, Supélec, Université Paris VI and Paris XI using a home-built extension of

a Digital Instruments Nanoscope III in contact mode (called “Resiscope”) [176]. The

tapping mode was used for making topographic images.

2.3.9. pH-potential-temperature measurements

This kind of measurement was used to better understand the aniline

polymerization process in the presence of the PA dispersion.

The changes of pH, potential as well as of the temperature during the

polymerization of aniline in the reaction mixture were recorded using Greisenger

electronic GMH 3530 (Digital pH/mV/Thermometer).

72

Chapter 3

ELECTROCHEMICAL

SYNTHESIS OF POLYANILINE

Chapter 3. Electrochemical synthesis

Introduction

It is known that the method of the electrochemical polymerization permits

materials with desired properties to be obtained under simply controlling

polymerization conditions (the potential and current values, time of polymerization,

material of electrode, etc.) [148, 177]. The other advantage of this method is the

possibility to avoid by-products of the process. Besides, this method is very useful when

we need to study the mechanism of the process. Up to now certain studies of the aniline

electrochemical polymerization process have been performed [1, 29, 30, 178]. But they

don’t give a complete picture of the mechanism and stages of the aniline polymerization

process especially in the polymer matrices.

In order to better understand the process, which takes place during the

electrochemical aniline polymerization in the PM we have preliminary investigated the

specificity of the electrochemical aniline polymerization on some bare electrodes

(platinum and SnO2-glass electrodes). The obtained results are discussed. Such an

approach makes it possible to understand and explain the aniline polymerization process

on the electrodes covered with the PM.

3.1. The peculiarities of the electrochemical formation and stability

of polyaniline on the surface of bare electrodes

Electrochemical synthesis of PANI on platinum and SnO2-glass electrodes was

carried out in potentiostatic, galvanostatic and cyclic voltammetry modes. All

measurements were performed in the aqueous solution of 1M HCl, containing 0.5M

aniline.

3.1.1. Polyaniline formation by cyclic voltammetry

Platinum electrode. The cyclic voltammograms of the aniline polymerization

on platinum electrode are demonstrated in Fig. 3.1a. The current increase at 0.6 V has

been observed even on the anodic branch of the first cycle. This increase is attributed to

the monomer (aniline) oxidation [163] and, respectively, to the formation of its cation-

radicals С6Н5NH.+. A continuous current increase of this peak and a shift of its potential

to the lower values are observed during further cycles. This can be connected with the

73


Figure 3.1. Cyclic voltammograms of the PANI formation on (a) platinum and (b)

SnO2-glass electrodes in the aqueous solution of 1М HCl, containing 0.5М aniline

(sweeping rate υscan = 100 mV/s)

(b) (a)

74


catalytical activity of PANI formed on the electrode surface [29] as well as with the

appearance of aniline oligomers (di-, three-, tetramers, etc.) which are easily oxidized

[163]. Thus, with the PANI accumulation the two redox couples appear on cyclic

voltammograms at potentials 0-0.2 V (the maximum of anodic current at 0.14 V and

maximum of cathodic current at 0-0.10 V) and at ∼0.7 V. These two redox couples are

attributed to the two-stage electrochemical process of the PANI oxidation-reduction

[22]. Visually, the appearance of the green film on the electrode surface corresponds to

this.

The first current peak increased continuously with successive potential scans,

indicating that the electroactive PANI layer is being deposited on the electrode surface

[179]. During the potential cycling, the cathodic current growth begins to decrease

gradually as compared to the anodic one by approximately 0.019 mA/cycle. In

accordance with the literature [33, 35, 180] this can be explained by the fact that during

the potential cycling till 0.8 V the overoxidation of formed PANI takes place because of

the subsequent chemical reactions of the PANI degradation. Such the degradation can

lead to the irreversibility of the anodic process and, respectively, to the decreasing of the

cathodic current values. This is, probably, connected with the fact that overoxidized

PANI is less electroactive and has lower conductivity value. The overoxidation makes

the possibility of the reduction of formed PANI difficult. The last fact leads to the

increase of the separation of the cathodic peak compared to the anodic one

approximately by 0.01 V/cycle due to an ohmic component. The decreasing of the

potential limit to 0.75 V permits to avoid the PANI overoxidation. Significant

separation of the cathodic and anodic peaks (∆Ер ∼0.19 V) testifies, probably, that the

aniline polymerization process is determined by the kinetics of the slow electrochemical

process [149].

The second redox couple (Е ∼0.7 V) (Fig. 3.1a) appears gradually on cyclic

voltammograms of the PANI formation. The development of this redox couple is

analogous to the first one: the current values increase with the potential sweeping; the

cathodic current growth decreases gradually in comparison with the anodic one

approximately by 0.14 mA/cycle; the cathodic peak potential is shifted during cycling in

relation to the anodic peak approximately by 0.06 V/cycle.

From the dependence of the first anodic peak current on the t1/2 for the different

sweep rates (Fig. 3.2) we can see that the current of this peak increases continuously

75


Figure 3.2. The dependence of the anodic peak current of the first redox process on the

t1/2 (a) and the dependence of the anodic peak potential on the cycling time (b).

Conditions of the measurements: aqueous solution of 1М HCl, aniline concentration

0.5М, on the platinum electrode at different sweeping rates νscan:

1 - νscan = 100 mV/s; 2 - νscan = 50 mV/s; 3 - νscan = 20 mV/s; 4 - νscan = 10 mV/s;

5 – νscan = 5 mV/s

t1/2, min1/20 2 4 6 8 10 12 14 16

Cur

rent

of t

he a

nodi

c pe

ak, m

A

0

5

10

15

20

25

30

35

Cycling time, min0 50 100 150 200

Pote

ntia

l of t

he a

nodi

c pe

ak, V

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

(b)

1

2

3

4

5

1

2

4

5

2

t1/2, min1/21 2 3 4 5 6 7 8

Cur

rent

of t

he a

nodi

c pe

ak, m

A

0.0

0.2

0.4

0.6

0.8

1.0 3

(a)

76


with √t which is indicative of an increase of the amount of formed electroactive PANI.

On the other hand, this fact testifies to the diffusion limitations of the process [148]. The

obtained diagrams (Fig. 3.2a) are characterized by the two well-defined modes with

different slopes. These regimes, probably, testified to the presence of two different

stages of the aniline polymerization process. On the first stage the current increase is not

very noticeable that is, probably, caused by the induction period of the polymerization

process. Hence, we can suppose that the first stage is connected with the formation of

the oligoaniline cation radicals [104]. The subsequent aniline oxidative polymerization

causes the change of the slope of the dependence I = f(t1/2) (Fig. 3.2a). The greater is the

quantity of the polymer deposited on the electrode, the higher is the rate of the polymer

formation. In this case the process is limited, mainly, by the counter ions diffusion

through the formed electroactive PANI film. Similar behaviour is observed in the case

of the polymerization of the aniline derivative (o-methoxyaniline) [88].

During the aniline polymerization process the first anodic peak potential is

continuously shifted to more positive values (0.35-0.4 V) (Fig. 3.2b). This can be

explained by the fact that the aniline oxidation takes place on the interface the PANI

film – solution and electrons transfer from aniline to the electrode through the PANI

layer. The growth of the PANI thickness and, respectively, of its resistance, leads to the

ohmic component increase. Obviously, the overoxidation of PANI makes a certain

contribution to this process too as it decreases the PANI conductivity.

The shape and behaviour of the obtained cyclic voltammograms are in good

agreement with the results obtained previously for the aniline electrochemical

polymerization in different solutions [37, 44, 178, 179]. Especially with the fact that the

rate of the PANI overoxidation and, respectively, conductivity and electroactivity losses

become higher during potential cycling to more than 0.8 V [35, 44, 178, 181]. From this

point the PANI oxidation becomes less reversible. Thus, during the potential cycling till

1V the middle redox couple at 0.45V appears on the cyclic voltammograms. This

couple is attributed to the PANI degradation process and to the formation of the

benzoquinone [33, 34].

The dependence of the formed PANI weight on the cycle number (reaction

time) was found (Fig. 3.3). The polymerization was finished at -0.2 V. The formed

PANI film was washed by the distilled water for 5 min, and then dried at the ambient

temperature (∼20 0C) for 3-4 hours up to the stable weight. The weight of the obtained

77


PANI film increases gradually as it is seen from Fig. 3.3. The dependence has the linear

character with the slope equal to 0.00125. The results make it possible to determine the

average PANI quantity which can be obtained on the platinum electrode during one

electrochemical cycle in the cyclic voltammetry regime. This value is 0.36

mg/(cycle⋅cm2) at the potential scan rate equal to 50 mV/s in the potential range from -

0.2 V to 0.8 V.

Cycles0 10 20 30 40 50 60 70

go

ee

,ig

htr w

lym

P

Figure 3.3. Dependency of the weight of the deposited polymer on the platinum

electrode on the cycle number (scan rate νscan = 50 mV/s) in the aqueous solution of 1М

HCl, aniline concentration 0.5М

SnO2-glass electrode. The investigation of the aniline polymerization process

on the SnO2-glass electrode was performed in order to obtain additional information

about this process. The cyclic voltammograms of the polymerization of aniline on this

electrode are demonstrated in Fig. 3.1b. These voltammograms are significantly

different from those obtained on the platinum electrode at the same potential scan rate

(Fig. 3.1a). Thus, on voltammograms obtained on the SnO2-glass electrode the second

redox couple is not observed. Instead, one broad redox peak couple can be seen which is

attributed to the electroactive PANI accumulation on the electrode surface. However,

the anodic potential peak shifts to more positive potentials when repeating potential

0.00

0.02

0.04

0.06

0.08

78


cycling - ∼0.62 V for 160 cycles (compare with ∼0.35 V for 160 cycles on the platinum

electrode). Probably, this fact can be connected with a rather high resistivity of the SnO2

layer. Therefore, higher potentials are necessary for redox processes to happen.

The higher current densities which are observed on the platinum electrode (Fig.

3.4a) in comparison with those observed on the SnO2-glass electrode (Fig. 3.4b) are also

in good agreement with this explanation. During the potential sweeping the smooth

PANI film is formed on the SnO2-glass electrode. The colour of this film changes

depending on the applied potential from yellow to dark green at potentials -0.2 V and

0.8 V, respectively.

3.1.2. Polyaniline formation in the potentiostatic mode

Dependencies of the current density on the polymerization time on platinum

and SnO2-glass electrodes at different applied potentials (0.6; 0.7; 0.75 and 0.8 V)

obtained in the potentiostatic mode are demonstrated in Fig. 3.5. These diagrams testify

to the delay of the current growth with the augmentation of the PANI amount on the

electrode surface. This effect is strongly revealed at higher potentials and on the SnO2-

glass electrode, which has high resistance. The shape of dependencies testifies to the

presence of the induction time. This period on the platinum electrode decreases at

potential increasing and is 25 min for Е = 0.6 V, 9 min for Е = 0.7 V, 1.5 min for Е =

0.75 V and less than 1 min for Е = 0.8 V. For the SnO2-glass electrode the picture is less

clear because of its high resistance, but the deceleration of the process is observed on

this electrode too because of the increasing of the system resistance.

It should be noted that at potentials higher than 0.7 V the inhomogeneous film

of dark green colour is formed (at 0.8 V this film is practically black – PANI is formed

in the PNA oxidation state) and at 0.6 V rather homogeneous green film is formed. This

is charged with the formation of PANI in the ES oxidation state.

3.1.3. Polyaniline formation under galvanostatic conditions

The peculiarities of the PANI electrochemical synthesis in the galvanostatic

mode on platinum and SnO2-glass electrodes are well illustrated by the Е = f(t)

dependencies (Fig. 3.6). From the represented data one can see that the electrode

potential initially shifted to the positive side and reached the maximum value (0.8 V and

higher) and then gradually decreased to a steady value (approximately 0.6-0.7 V). This

value, in turn, increased with the augmentation of current density and corresponded to

79


Figure 3.4. The dependence of the first anodic peak current density on the cycle number

on (a) platinum and (b) SnO2-glass electrodes in the aqueous solution of 1М HCl,

aniline concentration 0.5М, scan rate 100 mV/s

Cycles0 20 40 60 80 100 120 140 160 180

Cur

rent

den

sity,

mA

/cm

2

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Cycles0 20 40 60 80 100 120 140 160 180

Cur

rent

den

sity,

mA

/cm

2

0

1000

2000

3000

4000

(b)

(a)

80


Figure 3.5. The dependence of the current density on the polymerization time at different

applied potentials on (a) platinum and (b) SnO2-glass electrodes in the aqueous 1M HCl

solution, aniline concentration 0.5М:

1 - Е = 0.6 V; 2 – Е = 0.7 V; 3 – Е = 0.75 V; 4 – Е = 0.8 V

Polymerization time, min0 10 20 30 40 50 60

Cur

rent

den

sity,

mA

/cm

2

0.00

0.02

0.04

0.06

0.08

0.10

1


Cur

rent

den

sity,

mA

/cm

2

0

200

400

600

800

1000

1200

1400

1600

(a) 4

3

2

1

(b) 3

4

2

81


the formation of a green conductive PANI film on the electrode surface. The initial

overpotential which is characteristic of the electrosynthesis under such conditions is

associated with the formation of the PANI particles on the electrode surface [182]. The

tendency of the maximum augmentation is well observed at the current density

increasing. At very high current density values (more than 5 mA/cm2 on the platinum

electrode) there is no potential maximum and the film grows rather quickly. But,

because the potential under such conditions is higher than 1V, the formed film is very

inhomogeneous, overoxidized, very dark, almost black colour. The significant

increasing of the potential in the first minutes can be explained by the fact that the

aniline oxidation and then formation of PANI in the PNA oxidation state take place. For

the PNA formation the potential value which is much higher than 1V is needed. As it is

known [9, 183], PANI in the PNA oxidation state is the catalyst of the PANI formation

process. After the formation of this initial layer, the aniline polymerization takes place

at lower potentials (the main synthesis takes place at potentials approximately 0.6-0.7

V). At higher current densities (Fig. 3.6, curves 3 and 4) the electrode potential, because

of the very quick consumption of the aniline (depolarizer), reaches higher values. Thus,

at such high potentials the additional secondary reactions (overoxidation of formed

PANI, decomposition of the water and electrolyte anions) can occur in the system [148,

184].

The potential jump which is observed at 0.1 mA/cm2 in the case of the SnO2-

glass electrode and at 5 and 10 mА/cm2 on the platinum electrode can be explained by

the fact that potential exceeds 1V and the formed PANI layer is overoxidized and

becomes less electroactive and less conductive. Further electrochemical reaction and the

aniline polymerization are caused by the potential decreasing.

But, in spite of such secondary processes, a linear relationship between the

weight of obtained PANI and the current density under galvanostatic conditions (Fig.

3.7), as well as in the case of the PANI formation in the cyclic voltammetry mode (Fig.

3.3), was discovered. Thus, by varying the current density we obtained the PANI yield

of 0.028 g/C on the platinum electrode.

3.1.4. Electrochemical properties of the polyaniline film in the background

solution

The investigation of the PANI film in the background solution (solution of 1M

HCl) was performed in order to establish the behaviour of pure PANI under conditions

82


Figure 3.6. The dependence of the potential on the polymerization time at different

applied current densities on (a) platinum and (b) SnO2-glass electrodes in the aqueous

1М HCl solution, aniline concentration 0.5М:

a – 1 – і = 0.1 mA/cm2; 2 – і = 1 mA/сm2; 3 – і = 5 mА/сm2; 4 – і = 10 mА/сm2

b - 1 – і = 0.001 mA/cm2; 2 – і = 0.005 mA/сm2; 3 – і = 0.01 mА/сm2;

4 – і = 0.1 mА/сm2


Pote

ntia

l, V

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6


Pote

nt V

ial,

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

(a)

4

3 2 1

(b)

4

3 2

1

83


Figure 3.7. Dependency of the weight of the deposited polymer on the platinum

electrode on the anodic current density (depositing time 50 min) in the aqueous solution

of 1М HCl, aniline concentration 0.5М

of charge-discharge. To that end, after the polymerization, i.e. after the accumulation of

a certain PANI amount, the platinum electrode with deposited PANI (discharged at -0.2

V) was washed by distilled water and transferred to a 1M HCl aqueous solution for

studying its stability.

With the help of cyclic voltammetry we observed the well-defined peak at

approximately 0.3 V (Fig. 3.8), which corresponds to the reversible inclusion of the

protons [180]. It should be noted that the redox activity of the grown polymer decreases

during the second cycle and then becomes constant. According to [178, 185] such the

behaviour can be explained by the electrochemically induced ion-transfer processes

involving dopant ions in the grown polymer. Thus, during the potential sweeping from

-0.2 to 0.8 V, the PANI oxidation takes place. In order to neutralize the appeared

positive charge on the oxidized units of polymer chain the induction of anions to the

polymer chain and the pushing out protons take place. Such process results in the

increase of the polymer weight [30, 36]. This process is reversible: during the next

potential sweeping to the cathodic side the polymer loses its positive charge and that’s

Current density, mA/cm20 1 2 3 4 5 6

Poly

mer

wei

ght,

g

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

84


Figure 3.8. Cyclic voltammograms of the PANI-modified platinum electrode in the

background electrolyte (aqueous 1М HCl solution), νscan = 100 mV/s

85


why the anions transfer back to the solution and the protons again enter the polymer.

However, the complete removal of anions doesn’t take place because of transport

limitations and a certain portion of them thus remains in the polymer film. As a result,

during further sweeping to the anodic side less quantity of anions can introduce to the

film. As a consequence, the peak’s current value decreases as compared with the first

cycle. After 5 cycles the response is stabilised due to stable quantity of anions

introduced in the film.

The process of charge compensation by moving the anions is unexpectedly

displayed when changing the sweeping rate in different directions on the same sample

(Fig. 3.9). Specifically, from Fig. 3.9 it was found that for the same PANI film at

different sweeping rates the current of the first anodic peak was higher during the

gradual increasing of the scan rate from 20 mV/s to 100 mV/s (Fig. 3.9a and 3.10,

curves 3 and 4) than during the decreasing of the scan rate in the opposite direction –

from 100 mV/s to 20 mV/s (Fig. 3.9b and 3.10, curves 1 and 2). It means that during the

slow potential scan rates the electrochemical processes (charge transfer) take place at a

greater depth, i.e. all PANI layers take part in it, resulting in the appearance of the

additional peak at approximately 0.5 V (Fig. 3.9a). This peak is attributed to the

formation of overoxidized PANI [35]. On the contrary, this peak is not observed during

the potential cycling at scan rate decreasing (Fig.3.9b). Also, it should be noted that the

cathodic and anodic peaks potential and the potential difference between them are not

dependent on the scan rate in the studied range of rates.

A very good linear relationship between the first anodic peak and the scan rate

(Fig. 3.10) is found, indicating a surface-controlled redox process [148]. With

increasing the scan rate the magnitude of the peak current increased. Thus, the process

of the counter ions transfer to the polymer film and back – to the solution during the

oxidation-reduction of PANI takes place practically inside the PANI layer.

3.2. The formation of the polymer composite materials

Preliminary investigation of the electrochemical aniline polymerization was

necessary for determining the conditions and the peculiarities of such a process in the

PM.

It is obvious that the electrochemical polymerization in the PM can be realized

only under the condition of the matrix film swelling in the proper conducting media to

86


Figure 3.9. Cyclic voltammograms of the PANI-modified platinum electrode in the

background electrolyte (aqueous 1М HCl solution) in the stationary mode:

a – from 20 mV/s to 100 mV/s; b – from 100 mV/s to 20 mV/s

1 - νscan = 20 mV/s; 2 - νscan = 50 mV/s; 3 - νscan = 100 mV/s

87


Figure 3.10. The variation of the anodic peak current with scan rate (a) and with √υscan

(b) for the PANI film in the aqueous solution of 1M HCl:

1, 3 – first cycle;

2, 4 – second and next cycles

υscan1/2, (V/s)1/2

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Cur

rent

of t

he a

nodi

c pe

ak, m

A

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Scan rate υscan, V/s0.00 0.02 0.04 0.06 0.08 0.10 0.12

Cur

rent

of t

he a

nodi

c pe

ak, m

A

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

3

2 4

1

3

4 2

1

(a)

(b)

88


insure ionic conductivity. This means that the peculiarities of such the polymerization

depend not only on electrochemical characteristics of the system, but also on the

swelling degree of the matrix. So, we investigated the swelling kinetics of the PA-12

film in the reaction media and determined the time necessary to get constant value of

swelling. The same investigation was performed in the PVA matrix in order to

determine the influence of the PM structure.

It was found that the swelling process proceeds slower in the PA matrix as

compared with the PVA matrix (the saturation in which reaches in 150 min) (Fig. 3.11).

For all further researches we used films which were in the reaction media within this

period of time.

Figure 3.11. Swelling

kinetic of the PM in the

aqueous solution of 1М

HCl, containing 0.5М

aniline:

1 – the PA-12 film;

2 – the PVA film

The voltammograms which represent the PANI formation process on

electrodes, covered by PA-12 and PVA in the cyclic voltammetry mode are illustrated

in Fig. 3.12. As one can see, this process is similar to that on the bare electrode (Fig.

3.1): at potentials 0-0.2 V and 0.65-0.75 V two couples of redox peaks are formed, the

height (current) of which also increases with each successive cycle. However, the

presence of the PM film on the electrode surface leads to the decrease of the current

values (approximately by 1.13 mA/cycle for PA-12 and by 1.47 mA/cycle for PVA)

(Fig.3.13) and also to the peak potential shift to the anodic side (on the average by 0.013

V/cycle and 0.03 V/cycle in the case of PA-12 and PVA, respectively) (Fig.3.14). The

difficulty of the aniline polymerization process on the electrodes covered with the PM

films may be explained by increasing of the system resistance, as in the case of the PM

Time, min0 50 100 150 200

Swel

ling

degr

ee, %

0

1

2

3

4

5

6

1

2

89


Figure 3.12. Cyclic voltammogrames of the aniline polymerization on the surface of

covered with the PM platinum electrode from 0.5M aniline + 1M HCl solution

at υscan = 100 mV/s:

a – the PVA film;

b – the PA-12 film

(a) (b)

90


presence the monomer molecule should diffuse not only through the formed PANI

layer, but also through the film of the PM. Really, the specific resistance of the working

electrode with the film of PA-12 was found to be increased by 107 Ohm⋅cm and in the

case of the PVA film - by 44 Ohm⋅cm.

It was found that the rate of the polymerization process depends on the PM

structure. However, the character of the PM structure influence changes depending on

the cycle number and, respectively, on the polymerization time. Thus, during the first

cycling stage the currents of redox peaks, which characterize the rate of the PANI

formation, are less in the PA-12 matrix. However, with time the ratio of the rate

changes - the peak currents in the PA-12 matrix exceed current values for the PVA film.

The obtained dependences (Fig. 3.13) testify that the PANI formation goes easier on the

bare electrode, than on the electrode covered with the polymer films. However, on the

electrode covered with the PVA film this process takes place with more difficulties than

on the electrode covered with the PA-12 film. The obtained dependence of the peak

potential on the polymerization time (Fig. 3.14) also confirms these results.

One can suppose that the reason of such behaviour can be a different degree

and rate of swelling of the PM in the working solution, which, in its turn, influences the

charge transfer process in the polymer. From the analysis of the kinetics of swelling and

the PANI peak current values it follows that in the case of the PA-12 film this factor

(swelling degree and rate of swelling) is not decisive at the swelling degree of 0.5%

beginning since 11 min of the electrode being in the working solution (compare Fig.

3.11 and 3.13). As a result, the rate of the electrochemical aniline polymerization and,

respectively, the quantity of formed PANI in the PA-12 matrix exceed the rate of the

process in the PVA matrix (even in spite of the fact that the PVA film can be swelled to

much more degree (Fig. 3.11)). Such differences testify to more effective aniline

electrochemical polymerization process in the PA-12 polymer film.

The last observation is a very important evidence of the influence of the PM

structure and can be connected with the physicochemical interaction of aniline and

PANI with PA. Such supposition is in agreement with the literature results on the

formation of hydrogen bonds between the imine PANI groups and oxygen of amide

groups in PA-6 [142, 143] and also with the obtained dependence of the first anodic

PANI oxidation potential on the cycling time (Fig. 3.14).

91


Figure 3.13. The dependence of the first anodic peak current density on the

polymerization time on (a) platinum and (b) SnO2-glass electrodes at 100 mV/s in 1М

HCl aqueous solution, aniline concentration 0.5М:

1 – bare electrode;

2 – electrode, covered with the PVA film;

3 – electrode, covered with the PA-12 film

Polymerization time, min0 20 40 60 80 100 120 140 160

Cur

rent

den

sity

of th

e an

odic

pea

k, m

A/c

m2

0.0

0.1

0.2

0.3

0.4


Cur

rent

den

sity

of th

e an

odic

pea

k, m

A/c

m2

0

2

4

6

8

10

12

14

(b)

1 2

3

2

3

1 (a)

92


Figure 3.14. The dependence of the first anodic peak potential on the polymerization

time on (a) platinum and (b) SnO2-glass electrodes at 100 mV/s in 1М HCl aqueous

solution, aniline concentration 0.5М:

1 – bare electrode;

2 – electrode, covered with the PVA film;

3 – electrode, covered with the PA-12 film

Polymerization time, min0 20 40 60 80 100 120 140 160

Pote

ntia

l of t

he a

nodi

c pe

ak, V

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Polymerization time, min0 10 20 30 40 50 60 70

Pote

ntia

l of t

he fi

rst a

nodi

c pe

ak, V

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

(a)

(b)

1

2

3

1 2

3

93


3.3. Conclusions

On the whole, the study of the peculiarities of the aniline electrochemical

polymerization on the surface of both bare and covered with the PM electrodes allows

making the following conclusions.

1. Electrochemical behaviour of the PANI formation on the bare electrodes in

potential sweeping range from -0.2 V to 0.8 V is described by two redox transitions:

leucoemeraldine-emeraldine salt and emeraldine salt-pernigraniline. Continuous cycling

of the potential results in the growth of the PANI layer on the electrode surface, which

is displayed in the increase of current peaks corresponding to the transitions. Kinetics of

the process becomes slower because of the increase of the system resistance due to the

growth of the PANI layer thickness.

2. The PANI formation occurs in two stages: the first one is connected with the

polymer nucleation on the bare electrode and is limited by the monomer diffusion. The

second one is limited mainly by the counter ions transfer through the formed PANI

layer.

3. On the SnO2-glass electrode the aniline polymerization process is strongly

limited by the resistance of SnO2 layer. On this electrode only one redox couple is

observed, which is attributed to the leucoemeraldine-emeraldine transition. However,

the anodic potential peak shifts to more positive potentials when repeating potential

cycling and becomes broader.

4. Polymerization processes studied under the galvanostatic conditions allows

easy controlling of the PANI quantity on the electrode surface. But at high current

density there is a risk to produce overoxidized PANI. Unlike this, potentiostatic mode

insures using the potential window of stable PANI. Its formation under the constant

potential is described by three distinct regions on the current-time dependencies. In the

first region an initial drop in the current was observed, which might be attributed to the

slow adsorption of the monomer. This was followed by an increase in current (region

II), which was attributed to the nucleation and growth of a new electroactive PANI

phase on the surface of the electrode. As the reaction time increased, the current became

steady (region III).

5. The voltammetry response of formed PANI is directly proportional to the

weight of the polymer deposited. Therefore, the rate of the polymer formation is directly

94


proportional to the increase of current, which displays the quantity of PANI. Linear

relationships between the obtained PANI amount and the current density or cycle

number (i.e. reaction time) are discovered.

6. It was found that the mechanism of the aniline polymerization process on the

electrodes covered with the PM is similar to that on the bare electrodes, but hindered by

the system resistance increase. At the same time, the efficiency of the process of the

aniline polymerization depends on the matrix structure and is higher in the PA-12

matrix that can be associated with the physicochemical interaction of aniline and PANI

with the PA matrix, specifically, with the formation of hydrogen bonds.

The obtained results allowed concluding that the composite PM/PANI films

retain electrochemical properties of pure PANI. This gives some hopes to use these

composite materials in the fields, where conductivity and electroactivity of PANI are

needed (electrochromic and optical devices, sensors, etc).

On the other hand, it should be noticed that the method of electrochemical

polymerization does not allow obtaining the conductive composite materials in a large

scale. Such limitation is connected with the necessity to consume a lot of electrical

energy and to use large equipment. This moved us to investigate the chemical aniline

polymerization inside or in the presence of the PA matrix. This method opens good

production perspectives of the PA/PANI composite in a large scale.

95

Chapter 4

CHEMICAL ANILINE

POLYMERIZATION IN SOLID

AND WATER DISPERSED

POLYAMIDE MEDIA

Chapter 4. Chemical polymerization

Introduction

The formation of the PANI composite films by the electrochemical aniline

polymerization (see chapter 3) is a rather promising method to produce such composites

for use in small size devices (sensors, etc.) [186]. However, this method does not allow

obtaining the conductive composite materials in a large scale because of the complexity

of the technological process. Hence, the chemical polymerization method can be

alternatively considered as being more suitable for practise. So, in this chapter we have

discussed the process of the chemical aniline polymerization in the PA films as well as

in the PA dispersion.

To understand characteristics of the polymerization, the kinetic peculiarities of

this process were studied. This approach allows not only judging about the mechanism

of the processes in the systems but also permits realization of the effective controlling

of such processes under real conditions. The kinetic parameters of the process are

studied depending on the oxidant nature and on the type of the PM.

4.1. The swelling kinetics of the polyamide films in the reaction

media

As in the case of the electrochemical polymerization, the chemical one may be

realized in the PM only under the condition of the matrix film swelling in the monomer

solution and in the reaction media. This means that the structure and the thickness of the

conducting layer will depend on the swelling degree of the matrix. Thus, we

investigated the swelling kinetics of the PA-12 film in aniline as well as in the reaction

media to determine the time necessary to get certain value of swelling and to understand

the influence of the PM structure on the mechanism and kinetics of this process.

It was established that the swelling process of the PA-12 film in aniline went

on faster than in the PA-6 film. It is well seen from the fact that the swelling saturation

is reached in ∼3.5 hours for the PA-12 film and only after more then 15 hours for the

PA-6 film (Fig. 4.1a and 4.1b, respectively). But, on the other hand, the maximum

swelling degree is higher in the case of the PA-6 film - 16.5 wt.% in comparison with

13.8 wt.% for the PA-12 film. On one hand, this fact may be explained, probably, by the

different hydrophilicity of the PM due to a different ratio of polar/nonpolar groups.

96


Figure 4.1. Swelling kinetic curves of the PA films in aniline:

a – the PA-12 film;

b – the PA-6 film

Time, hours0 20 40 60 80 100

Swel

ling

degr

ee, w

t.%

0

2

4

6

8

10

12

14

16

18

20

(b)

Time, hours0 2 4 6

Swel

ling

degr

ee, w

t.%

0

2

4

6

8

10

12

14

16

(a)

97


Indeed, the elementary chain of PA-12 consists of 11 aliphatic (CH2) groups,

whereas in a molecule of PA-6 there are only 5 such groups (Fig. 2.1). On the other

hand, different degree of crystallinity and, correspondingly, different free volume

amount of the used PA films can be also considered as important factor. This difference

was confirmed with the help of DSC measurements (Fig. 4.2). According to the

equation (2.9), the degree of crystallinity was calculated by the fusion enthalpy of the

polymer films (Table 4.1). It was found that the degree of crystallinity is higher in the

case of the PA-6 film when compared with that of the PA-12 film. So, as the PA-6

matrix has higher crystallinity degree, it contains less volume of the amorphous phase

than the PA-12 one. Additionally, the higher maximum swelling degree in the PA-6

film may be also explained by the fact that aniline molecules can form hydrogen bonds

with amide groups of PA, i.e. some physicochemical interaction of aniline with the PM

takes place. And as the quantity of the polar amide groups are higher in the case of the

PA-6 (Fig. 2.1), so the quantity of hydrogen bonds is higher too. Thus, the film of PA-6

can hold more aniline molecules compared with the film of PA-12. It should be

mentioned that for further kinetic investigation we used films which were set in aniline

within different periods of time in order to receive films with different degree of

swelling.

Figure 4.2. DSC

thermographs of the PA

virgin films

Also, we investigated the swelling kinetics of the PA films in the reaction

media we used, i.e. in distilled water, in aqueous HCl solution (1M) and in

Temperature, 0C-100 -50 0 50 100 150 200 250 300

Hea

t Flo

w

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

PA-6 PA-12

Exo

98


Table 4.1. Degree of crystallinity of the PA films

Type of PA

Parameter

PA-6

PA-12

∆Hc, J/g 60.7 48.6

∆H°, J/g 160 [187] 233.5 [188]

Xc, % 37.9 20.8

aqueous acidic (1 M HCl) solution of 0.1 M (NH4)2S2O8 (Fig. 4.3). As one can see in

the case of these solutions the situation is the same – the film of PA-6 (Fig. 4.3a)

reaches much higher values of the saturation in comparison with the film of PA-12 (Fig.

4.3b). And this fact is in good agreement with the explanation given above – that the

swelling kinetics goes faster in the case of the PA-6 film because of its structure (it

contains more polar groups than PA-12). This resulted in more hydrophilic nature of the

former.

4.2. The influence of the oxidative media on the polymerization

process

The determination of kinetic peculiarities of a process in combination with the

simultaneous identification of the forming products is one of the most reliable

approaches to study the reaction mechanism. This approach we applied to study the

chemical aniline polymerization process, which is accompanied by the optical density

system changes [37, 44, 108]. We used this fact to monitor the aniline polymerization

process by UV-Vis spectroscopy, since the intermediates as well as the resulting

polymer product are known to be coloured [108, 119]. As changing parameter here we

used the oxidizer nature – we used aqueous acidic solutions of APS and chlorine water

solution.

The UV-Vis spectrum of aniline is characterized by the presence of bands only

in the ultraviolet region (180-230 nm). These bands are caused by the electron

transitions of the benzene ring and of the amine group [54]. It was found that due to the

interaction of aniline with an oxidant in the solution the substantial spectral changes

took place owing to the PANI formation (bands at 315, 450 and 875 nm) [119]. In the

99


Figure 4.3. Swelling kinetic curves of the PA-6 (a) and PA-12 (b) films in the different

media:

1 – distilled water;

2 – aqueous HCl solution (1M);

3 – 0.1M (NH4)2S2O8 + 1M HCl solution

Time, hours0 5 10 15 20 25

Swel

ling

degr

ee, w

t.%

0

2

4

6

8

10

12

14

16

18

Time, hours0 5 10 15 20 25

Swel

ling

degr

ee, w

t.%

0.0

0.5

1.0

1.5

2.0

2.5

(a)

(b)

1

2

3

1

2 3

100


PA matrices similar changes are observed. Indeed, as we can see from Fig. 4.4 – 4.6, in

a few minutes after the beginning of the polymerization process in the PA-12 matrix the

PANI bands appear and continuously grow.

Visually this process in the PM swelled in aniline and immersed in the oxidant

aqueous solution begins from the formation of the blue regions at the matrix surface.

These regions rapidly change their colour to green one, merge gradually in the course of

polymerization and colour in green all the surface of the film sample. Such colour

changes testify that the PANI formation in the PM is realized through the typical stages

of the pernigraniline (Fig. 1.1) and emeraldine salt (Fig. 1.2) formation. However, the

UV-Vis spectra measured during the polymerization process do not show the stage of

the pernigraniline formation probably because of the big gap (∼3-10 min interval)

between the spectra (Fig. 4.4 – 4.6).

Figure 4.4. UV-Vis spectra of

the aqueous solution of

1 – 2.0 min; 2 – 5.0 min; 3 –

28.1 min; 8 – 32.7 min; 9 – 4

400

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

2.5

3.0

9

01 2

the aniline polymerization process in the PA-1

water chlorine solution ([Cl-] = [HOCl] = 0.003

7. 7 min; 4 1. in; 5 – 17.4 min; 6 – 20.5

5.5 10 – 76.3 min; 11 – 128.6 min; 12 – 1

Wavelength, nm500 600 700 800 900

1

23 4

5 6

7

8

min;

8 m – 1

101

1
1
1

2 matrix in

N):

min; 7 –

52.1 min


Figure 4.5. UV-Vis spectra of the aniline polymerization process in the PA

the aqueous solution of 1M HCl + 0.1M (NH4)2S2O8:

1 – 1.5 min; 2 – 4.8 min; 3 – 7.5 min; 4 in; 5 – 12.7 min; 6 – 1

7 – 18.3 min; 8 – 22.8 min; 9 – 26.1 min; 10 – 29.4 m – 32.8 min; 12

13 – 46.4 min; 14 – 50.7 min; 15 – 80.3 m – 119.1 min

The UV-Vis spectra of the PANI formation in the PM measured

oxidation systems (chlorine water solution, solutions of APS in hydr

sulphuric acids) reveal the presence of two bands (at 320-340 nm and at

and the shoulder at 410-440 nm. Such features of the spectra testify to the

PANI in the emeraldine salt oxidation state as in the case of the aniline po

in the solution [189]. The band at 320-340 nm can be assigned to π-π∗

benzene rings, the shoulder at 410-440 nm and the broad band at 700

obviously referred to the polaron transition [6]. Intensity of these ban

gradually in the course of the polymerization (Fig. 4.4 – 4.6). However, w

intensity and the position of the maximum change depending on the oxidati

Specifically, in case of the water chlorine solution with a small c

of Cl2 (0.003 N), the increase of the band at ∼750 nm, which is attributed t

absorbance [181], is observed (Fig. 4.7, curve 1). The gradual increase of th

– 10.1 m

in; 11

in; 16

Wavelength, nm400 500 600 700 800 900

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

2.5

1

12 13

14 15

16

102

91
011
-12 matrix in

5.3 min;

– 36.2 min;

in all studied

ochloric and

700-750 nm)

formation of

lymerization

transition of

-750 nm are

ds increases

ith time the

on system.

oncentration

o the polaron

is maximum

3 4 5 6 7 8

2


Figure 4.6. UV-Vis spectra of the aniline polymerization process in the PA

the aqueous solution of 1M H2SO4 .1M H4)2S2O8:

1 – 1 min; 2 – 4.5 min; 3 – 7.8 min; 4 – 11.1 min; 5 – 14.4 m ; 6 – 17

7 – 20.6 min; 8 – 23.7 min; 9 – 27.1 min; 10 – 30.5 min; 11 – 3 mi 2 –

13 – 68.8 min; 14 – 100.5 min; 15 – 138 min; 16 – 157 min; 17 – 16

is accompanied by its widening to the blue side of the spectrum. Such beha

explained by an augmentation of the formed polymer quantity as we

bipolaron appearance [181] and the overoxidation processes. In approxim

fter the beginning of the polymerization process some deceleration of th

observed followed by the stopping of this peak growth (Fig. 4.4 and 4.7, c

can be explained by deep oxidation (and even overoxidation) of the

ontaining layers, which contact with the oxidant solution.

In the case of APS water solution in hydrochloric acid as an

increase of optical density at 750 nm is also observed during the first m

reaction (Fig. 4.5 and 4.7, curve 2). However, in this case the decele

polymerization process is observed later (at the 70-th min)

+ 0 (N

in

n; 1

a

c

Wavelength, nm400 500 600 700 800 900

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

4

9

0

23

45

67

1

103

1 2 3 56 7 8

1
1
1
1
1
1
1
1
-12 matrix in

.5 min;

37.5 min;

8 min

viour can be

ll as by the

ately 30 min

e reaction is

urve 1). This

upper PANI

oxidant, the

inutes of the

ration of the


spectra of the polymerization process in APS solution in different acids (Fig. 4.7, curves

accompanied by the reaction of chlorine hydro

than the charged S2O82- anions. As a result, the aniline polymerization in the chlorine

containing media, i.e. in the presence of HOCl, penetrates to the deeper layers of the

PM film. In the sulphuric acid medium with only one oxidant (ASP) this process is

localized in a very thin subsurface layer of the dielectric matrix due to the limitation of

Figure 4.7. Kinetic curves of the aniline polymerization process in the PA-12 matrix in

different oxidation media:

0 18020 40 60 80 100 120 140 160

Polymerization time, min

1 – chlorine water solution, λmax = 750 nm;

2 – 0.1M (NH4)2S2O8 + 1M HCl, λmax = 750 nm;

3 – 0.1M (NH4)2S2O8 + 1M H2SO4, λmax = 670 nm

And, in the oxidation system of the aqueous solution of APS in sulphuric acid

not only stopping of the increase of the optical density is observed, but even its sharp

dropping (Fig. 4.6 and 4.7, curve 3). The observed difference between the UV-Vis

2 and 3) may be explained by the influence of the background electrolyte (HCl or

H2SO4) on the PANI formation process. It is known that in the aqueous solution of

hydrochloric acid in the presence of APS the formation of chlorine proceeds. This is

lysis resulted in the formation of HOCl

being an effective oxidant in the aniline polymerization in the PM [152]. The

nondissociated molecules of HOCl can penetrate to the dielectric matrix more easily

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

2.5

2

3

1

104


S2O82- penetration and is accompanied, obviously, by the overoxidation and degradation

of PANI formed inside the upper matrix layers. These results in higher decrease of

optical density of the formed composite (Fig. 4.6 and 4.7, curve 3) than in the case of

HCl background electrolyte (Fig. 4.4 and 4.7, curve 1).

It should be noticed that in the sulphuric acid solution the maximum of the

absorbance is shifted to the shorter wavelength region unlike other oxidation systems

(670 nm in comparison with 750 nm, respectively). The shift to 670 nm, i.e. the blue

shift, can be the evidence of the polymer formation with the shorter conjugation length.

4.3. Evaluation of the structure of the surface composite films

In order to study the kinetic parameters (e.g. reaction orders) of the chemical

aniline polymerization process and to accomplish the calculations of the PANI

formation process in the PM, it is necessary to determine the structure of the obtained

omposite film and the thickness of the conducting layer.

The structure and thickness of the composite surface films were successfully

determined by using Resonance Raman spectrometry (see section 2.3.4).

Preliminary we measured the virgin PA-12 film, pure doped PANI powder and

omposite film under the same experimental conditions using the confocal micro-

Raman spectrometry (Fig. 4.8). Spectra presented in the figure clearly demonstrate the

enhancement of the bands o A the bands of PA-12 are very weak when

recorded under the same conditions. As one can see the spectrum of the composite film

(Fig. 4.8, curve 3) do not contain any well-defined band c acteristic of the PA-12

. Due to the enhancement of the band intensity originating from PANI, the signals

om PA will be considered negligible for the complete analysis. On the other hand, one

can notice that the spectra of the composite film and the pure doped PANI powder are

similar (Fig. 4.8, curve 2 and 3). This observation is in good agreement with the results

f the electrochemical aniline polymerization inside the PM (chapter 3). It was shown

that the composite material displays the features of PANI obtained under “free”

onditions.

Hence, taking into consideration the advantage of the confocal system and the

enhancement of the PANI vibrational bands under the resonant conditions we were able

determine the thickness of the PANI containing layer (Fig. 4.9). So, the dependence

of the integrated Raman intensity in the wavenumber region 1150-1670 cm-1 on a

c

c

f P NI while

har

film

fr

o

c

to

105


sample p

also plotted versus the Z

displacem nt (Fig. 4.10b).

Fig. 4.10 clearly shows that the distribution of PANI in the PM is similar for

the PA-6 and the PA-12 films. The obtained profile of the integrated Raman intensity is

symmetric and, therefore, proves that the PANI containing layer can be considered as a

homogeneous layer of a definite thickness. The experimental points were fitted by

applying a Gaussian law. The distance d (Fig. 4.10) was obtained as a half of the width

Figure 4.8. Raman spectra collected under the same experimental conditions

(λ = 514.5 nm, t = 300 s, p = 0.04 mW):

1 – the PA-12 film;

2 – the PANI powder in the form of ES;

3 – the composite PA-12/PANI doped film

Wavenumber, cm-1

Ram

an in

tens

ity, a

.u.

osition is shown in Fig. 4.10.

The same approach and measurements were used to analyze the thickness of

the PANI containing layer formed in the PA-6 film. The evolution of the integrated

Raman intensity of the PANI characteristic bands was

e

500 1000 1500 2000 25000

500

1000

1500

2000

2500

3

1

2

106


Figure 4.9. Schematic presentation of the Raman depth analysis (a) and Raman

spectra collected on the doped composite film based on PA-12 (b):

1 – 5 - different sample positions below the objective

Wavenumber, cm-1500 1000 1500 2000 2500

Ram

an in

tens

ity, a

.u.

0

500

1000

1500

2000

2500

5

3

4

(a)

(b)

2

1

PANI containing layer

Insulating layer

12345

Z

objectiveobjective

107


1.4e+5

1.6e+5

1.8e+5

Figure 4

.10. Evolution of the integrated Raman intensity of the PANI vibrational bands

in the wavenumber region 1150-1670 cm-1:

a – the PA-12 matrix; b – the PA-6 matrix

Sample position Z, µm-2 0 2 4

3 (a)

Inte

grat

ed R

aman

inte

nsity

, a.u

.

6.0e+4

8.0e+4

1.0e+5

1.2e+5

d 4 2

4.0e+4

2.0e+4

5 0.0

1

Sample position Z, µm-4 -2 0 2 4 6 8

Inte

grat

ed R

aman

inte

nsity

, a.u

.

-2.0e+4

0.0

2.0e+4

4.0e+4

6.0e+4

8.0e+4

1.0e+5

1.2e+5

(b) 3

2 4

5 1

d

108


of the profile pea

merization we used

. Correspondingly, we can

posite film. This layer

retards diffusion of the oxidant accordingly, slows

down the form . This fact is in a good agreement

with the resu merization in the

PA-6 (Nylon) film

ANI

) of the

) is

found m

ion

of the oxidant solution od agreement with the

DSC measurements (Fig. 4. of the PM swelling kinetics in the

aqueous reaction media (Fig.

The obtained values of the

lations of the PANI yield in the composite films

and to de

(n) thickness δ, µm

k. Obtained results show that the method of the aniline poly

resulted in a double-layered conducting composite film

suppose that the process runs in an external layer of the com

solution into the interior one and,

ation of PANI inside the polymer film

lts obtained by Byun and Im [143] for the aniline poly

.

It should be mentioned that such a layered architecture is similar in the PA-6

and PA-12 films since in both cases we obtained the same depth profile (Fig. 4.10).

On the contrary, the thickness of the conducting layer strongly depends on the

structure of the PM as one can see from Table 4.2. The thickness δ of the P

containing layer was determined by taking into account the refractive index (n

PM and the full-width at half-maximum of the Raman profile (d):

δ = d ⋅ n (4.1)

The thickness of the conducting layer in the case of the PA-6 film (4.7 µm

ore than twice as much as for the PA-12 film (2 µm). This result can be

explained by the more hydrophobicity of PA-12 (Fig. 4.3), which reduces the diffus

into the PM. This explanation is in a go

2) and with results

4.3).

thickness of the conducting PANI containing layer

(Table 4.2) was used in further calcu

termine the kinetic parameters of the polymerization process.

Table 4.2. Results of the Raman depth analysis

Matrix

Distance d, obtained from

Raman depth analysis, µm

Refractive index Conducting layer

PA-6 3.1 1.53 [190] 4.7

PA-12 1.3 1.54 [190] 2

109


s solutions is rather

ecause of the

precip e rate of aniline

consum erization rate are

used 115]. Unlike this, the slow rate of the aniline polymerization process in the

transparent PM allows controlling the process in situ with the help of the UV-Vis

spectrometry as it was shown in the previous section.

As it follows from the known kinetic studies of the aniline polymerisation

[115, 191-194], under all other equal conditions the quantitative characteristics of the

t concentration

poly e thin surface layer of the PM.

.4.1. Polymerization process in the PA-12 film

4.4. Kinetic peculiarities of the polyaniline formation in the

polyamide matrix

The kinetic investigation of the PANI formation in aqueou

difficult because of the fast aniline polymerization process and also b

itation of PANI [115, 191]. Therefore, in many cases either th

ption is registered [191] or aniline derivatives with less polym

[

process depend on the aniline concentration in the matrix and the oxidan

in the solution. So, we investigated an influence of both factors on the kinetics of the

merization process inside th

4

As it is shown above, the final composite polymer film has a layered structure.

The external two layers are conducting PANI containing layers and the internal layer is

the pristine PA. The thickness of the PANI containing layer was determined by Raman

spectrometry (see section 4.3) and was found to be dependent on the structure of the

PM.

We postulate that aniline is evenly distributed only inside the PM and does not

go to the oxidant solution during the polymerization process. This is confirmed by the

fact that neither aniline nor products of its oxidation were registered in the solution. The

range of the used aniline concentrations in the polymer film is between 0.56-1.36 M and

of the used oxidant (APS) concentrations in the solution are 0.1-0.2 M.

The obtained kinetic profiles were measured for the exciton band at 750 nm.

As one

defined S- erization

in the solution [183, the existence

of three intermediate stages, which can be named as the induction period, the

can see from Fig. 4.11, these absorbance-time dependencies have the clearly

shape character, which is typical for the autocatalytic aniline polym

195-197]. Such character of the curves clearly reveals

110


intermediate propagation stage and the deceleration stage [193, 198]. The same

behaviou of the po erization process wa observed for the electrochemical method

of the PANI formation as it was noticed previously (chapter 3).

e difference between the investigated systems may be clearly seen in the

dependence of the induction period, the slope and the character of the kinetic curves on

the reagent concentrations. Specifically, in the case of the aniline polymerization in the

water solution of DBSA Madathil et al [ displa d data that allowed us to conclude

that the induction period decreased approximately by a factor of eight when doubling

the amount of APS. Slope of the kinetic curve part, which corresponded to the

propagation stage, kept practically unchanged. This observation confirmed the

independence of the rate of the propagation stage on the oxidant concentration in this

solution system. In contrast to this, for the aniline polymerization in the PM much less

change of the induction period is observed, but a rise of the slopes both of the induction

period and the propagation stage parts of the kinetic curve are noticed when increasing

the oxidant concentration (Fig. 4.11a). But the rate of the propagation step is strongly

affected by the oxidant concentration that can be seen from the change of the slope of

the kinetic curves (Fig. 4.11a).

At the same time the increase of the aniline concentration in the PM from 0.56

M to 0.81 M also did not noticeably influence the induction period duration (Fig.4.11b,

curves 1-3). But such a concentration increase resulted in more than two-fold change

(growth) of the slope of the kinetic curve part corresponding to the propagation stage

(i.e. its rate). The further increase of e aniline concentration to 1.12 M resulted in the

change of induction period time as well as curve shape (Fig. 4.11b, curve 4). Such

behaviour displays probably the change in a condition of the PA-12 film swelling.

Finally, at 1.36 M of the aniline concentration in the PM the shape of the kinetic curve

dramatically changed testifying to a significant decrease of the induction period and

also to the increase of the polymerization rate without the termination step (Fig. 4.11b,

curve 5), which is observed at lower aniline concentrations (Fig. 4.11b, curve 1-4). On

the contrary, the decreasing of the duration of the induction period was obtained with

the increase of aniline concentration (Fig. 4.11b).

period duration on n and also to the

independence of the p trations in the PA-12

r lym s

Th

193] ye

th

The kinetic curves obtained testify to the actual independence of the induction

the oxidant concentration in the solutio

ropagation stage rate on the aniline concen

111


1.0

1.5

2.0

4.11. Kinetic cu of the PANI form PA-12 film at 750 nm at

rent (a) aniline ( 4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 0.5582 M)

concentrations:

a - 1 - 0.1 M; 2 - 0.15 M; 3 - 0.17 M; 4 - 0.2 M

b - 1 - 0.56 M; 2 - 0.66 M; 3 - 0.81 M; 4 - 1.12 M; 5 - 1.36 M

4.11. Kinetic cu of the PANI form PA-12 film at 750 nm at

rent (a) aniline ( 4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 0.5582 M)

concentrations:

a - 1 - 0.1 M; 2 - 0.15 M; 3 - 0.17 M; 4 - 0.2 M

b - 1 - 0.56 M; 2 - 0.66 M; 3 - 0.81 M; 4 - 1.12 M; 5 - 1.36 M

Figuregure rvesrves ation in the ation in the

diffediffe С(NHС(NH

Polymerization time, min0 100 200 300 400

1

2

4 3 (a)

Abs

orba

nce

0.5

0.0

1

2

3 4

Abs

orba

nce

Polymerization tim ine, m0 100 200 300 400

0.0

0.5

1.0

1.5

2.0

2.5

4

(b) 5

3 2

1

112


film not higher than 0.81 M.

e typical

an the solution systems. For the polymerization in the solution

authors [ 15, 191, 192] proposed a semi-empirical kinetic model, in which aniline is

consume

These observations strongly differ from known kinetic results of th

iline polymerization in

1

d in a slow, homogeneous reaction with the oxidant (in fact this is an induction

period), followed by much faster heterogeneous reaction with the PANI precipitated

phase (an autoacceleration step). This enabled them to propose the following empirical

rate equation:

[ ] [ ] [ ] [ ] [ ]PANkAPSANkdtANd

⋅+⋅=− , (4.2)

where [AN], [APS] and [P] are the initial concentra

21

tions of aniline, APS and PANI,

correspo

ends on the aniline

concentrations and the PANI quantity formed in dispersed form in the solution phase

[115, in the case of the aniline

polymer

laniline) [115]. For the last case Sivakumar et al [115] reduced a kinetic equation

for the p

ndingly. k1 and k2 are constants of the homogeneous and heterogeneous stages,

respectively. In turn, k2 is an observed complex constant including a contribution from

the specific surface area of the precipitated PANI.

As one can see, the homogeneous stage is determined by both aniline and

oxidant concentrations, while the heterogeneous one dep

191, 192]. However, as we considered above,

ization in the PM all the formed products are located in the matrix. So, we can

say that the behaviour of the aniline chemical polymerization in the PM is more close to

the chemical polymerization of the aniline derivative, e.g. N-methylaniline (NMA),

which results in the formation of the soluble in the polymerization medium poly(N-

methy

olymerization rate Rp to the simplified one constant equation:

[ ] [ ]PDSNMAkR p ⋅= 2 , (4.3)

where [NMA] and [PDS] are concentrations of the NMA and potassium peroxodisulfate.

So, taking into account the resemblance of the polymerization of aniline

derivatives and the aniline polymerization in the PM and the fact that the kinetics was

measured by changes of the absorption of the polymer at 750 nm, we found it also

possible to use the similar simplified equation to describe the process of the PANI

formation in the PA matrix:

[ ] [ ]yxp APSANk

dtdAk

dtdPR ⋅=== 750 , (4.4)

113


114

where dtdP is the rate of the PANI formation, 750k is a coefficient of proportionality

between the content of PANI formed in the external subsurface layer [P] and its

absorptio

concentration was

k

2 and 3). A straight line is obtained

with the

n A at 750 nm. k is the observed constant; x and y are reaction orders with

respect to the aniline and APS, respectively.

In order to get suitable conditions for obtaining both reaction orders of the

polymerization process, a kinetic study was carried out with different concentrations of

oxidant in the solution at the constant aniline concentration in the PM (Fig. 4.11a) and

vice versa – with different concentrations of the aniline in the PM at a constant

concentration of the oxidant in the solution (Fig. 4.11b)

To evaluate the reaction orders we took into account the layered structure of

the composite film and the fact that during kinetic measurements the conducting layer

was formed at both sides of the pristine PM as the film was placed into the oxidant

solution. Correspondingly, the reaction zone in the case of the PA-12 film is the layer

with the thickness of 4 µm (2 µm at each side of the PM).

Thus, in order to determine the reaction order x the growth rate is calculated

from the slope of the kinetic curves on Fig. 4.11b. Since the oxidant

ept constant and only the concentration of aniline varied, the double logarithmic plot

of the growth rate V0 versus the aniline concentration [AN] is shown in Fig. 4.12a. A

straight line was obtained with the slope equal to 2.8. This indicates that the PANI

formation is 2.8-order with respect to the aniline, i.e. higher than in the case of the

aniline polymerization in the solution (equal to 2 in the case of using potassium

peroxomonosulphate [115]). Such a high reaction order testifies, obviously, to some

complication of the interaction between the localized monomer (aniline) in the PM and

APS in the solution.

To determine the reaction order with respect to APS the slope of kinetic curves

of the PANI formation in the PA-12 matrix at λ = 750 nm obtained at the fixed aniline

concentration in the film and with different oxidant concentrations in the solution (Fig.

4.11a) is counted. Again, the double logarithmic plot of the polymerization rate versus

concentration of APS is plotted (Fig. 4.12b, curve

slope equal to 1.6, which is again higher than for the aniline polymerization in

the solution (equal to 1 [115, 194]). It should be noted that we carried out measurements

for different aniline concentrations in the PM and the fixed concentration of APS in the


e

concentration of the aniline (a) and uring the aniline polymerization in

2, 3 - (NH4)2S2O8 + 1M HCl (Сaniline = 0.56 M (2); Сaniline = 1.12 M (3))

(b)

lg[AN·104]3.7 3.8 3.9 4.0 4.1 4.2

lg[V

0·104 ]

Figure 4.12. Double logarithmic plot of the polymerization rate V0 on th

the oxidant (b) d

the PA-12 film:

1 – 0.1M (NH4)2S2O8 + 1M HCl

0.4

.6

.8

1.0

1.2

.4

.6

.81

(a) 1

1

0

0

1

lg[APS·104]2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

lg[

0·10

V4 ]

0.4

6

8

0

1.2

1.4

1.6

1.8

2.0

1.

0.

0.

(b)

3

2

115


solution (Fig. 4.12b, curves 2 and 3). This was made in order to check that the reaction

order do

cid is performed (Fig. 4.13). It is established that the dependency of the

polymerization rate on the conversion degree has parabolic character, which is the

characteristic of the autocatalytic reactions [29, 196]. In an autocatalytic reaction the

yield in the

PA-12 film

Oxidant

es not depend on the aniline concentration in the PM. It was found that in both

cases the reaction order is 1.6. Probably, the fractional reaction order with respect to the

oxidant (APS) may be explained by the formation of the other additional oxidant HOCl,

which is weakly dissociated and easily penetrates into the PM [152]. This fact can lead,

in turn, to the changes in the mechanism of the polymerization [189].

The differentiation of the kinetic curves of the PANI formation in the

hydrochloric a

rate in the beginning of the process is low because a little product is present. It increases

to a maximum as a product (PANI) is formed and then again drops to a low value as a

reactant (aniline) is consumed. The obtained curves show the same trends as for the

aniline polymerization in the solution [198].

The obtained results confirmed that the concentration of the oxidant as well as

the concentration of aniline influences the polymerization rate of the reaction. Thus, the

aniline concentration in the PM influences the initiation and propagation rates – when

increasing the aniline concentration the time of the induction period decreases and the

polymerization rate increases (Fig. 4.13a). On the other hand, when increasing the APS

concentration the rate of polymerization increases too (Fig. 4.13b).

Taking into account the thickness of the conducting layer we also evaluated

the yield of PANI in the PM (Table 4.3 and 4.4).

Table 4.3. Effect of the oxidant (APS) concentration on the PANI

General General

concentration,

M

Aniline

concentration,

M

quantity of APS

in the solution,

mol

quantity of

aniline in the

PM, mol

Yield of the

PANI

formation, %

0.1 0.56 3·10-4 6.03·10-7 49.3

0.15 0.56 -4 -74.5·10 6.03·10 63.6

0.17 0.56 5.1·10-4 6.03·10-7 90.0

0.2 0.56 6·10-4 6.03·10-7 91.9

116


117

Figure 4.13. The rate of the PANI formation versus the conversion degree of the

polymerization reaction in 1 M HCl aqueous solution:

a – at the fixed oxidant concentration (С(NH4)2S2O8 = 0.1 M);

b – at the fixed aniline concentration (Сaniline = 0.56 M)

Conversion degree, %0 20 40 60 80 100

Poly

mer

izat

ion

rate

V0·1

04 , mol

/l·m

in

0

20

40

60

80

1000.1 M APS0.15 M APS0.17 M APS0.2 M APS

(b)

(a)


Poly

me

·104 ,

rizat

ion

rate

V0

mol

/l·m

in

0

20

04

12

180

60

80

100

0

140

160

0.56 M aniline0.63 M aniline0.66 M aniline0.81 M aniline1.12 M aniline1.36 M aniline

(a)


Table 4.4. Effect of the aniline concentration on the PANI yield in the PA-12 film

Aniline

concentration,

M

Oxidant

concentration,

M

General

quantity of APS

in the solution,

mol

General

quantity of

aniline in

the PM, mol

Yield of the

PANI formation,

%

0.56 0.1 3·10-4 6.03·10-7 49.3

0.63 0.1 3·10-4 6.83·10-7 53.9

0.66 0.1 3·10-4 7.13·10-7 60.9

0.81 0.1 3·10-4 8.77·10-7 57.2

1.12 0.1 3·10-4 1.21·10-6 53.9

1.36 0.1 3·10-4 1.47·10-6 47.7

As one can see, the yield of the reaction of the PANI formation in all studied

solutions does not exceed ∼65%. Only at high oxidant concentration (0.17 and 0.2 M),

i.e. under the conditions when the more quantity of HOCl is formed, the yield of PANI

approaches ∼92

Nevertheless, the PANI amount for ed in all the cases is sufficient to provide

the value of conductivity necessary for the application of these films as antistatic

materials (Table 4.5). The conductivity of the virgin PA-12 film is given for

comparison.

Table 4.5. Influence of the polymerization solution on the conductivity of the composite

PA-12/PANI films

Polymerization solution Conductivity, S/cm

%.

m

Chlorine water solution 1-2⋅10-5

0.1M (NH4)2S2O8 + 1M HCl 5-8⋅10-5

0.1M (NH4)2S2O8 + 1M H2SO4 3-7⋅10-7

Pure PA-12 10-13-10-14

118


4.4.2. Influence of the polymer matrix structure

The investigation of the aniline polymerization process in the PA-6 polymer

film was performed in order to obtain additional information about this process and in

order to elucidate the influence of the PM structure on the polymerization process.

Kinetic curves of the aniline polymerization inside this PM are presented in

Fig. 4.14. Obtained curves are significantly different from those obtained during the

polymer

, as a result, by the deeper penetration of

the oxid

seen by changes of

the film

aximum is observed, as can be seen

in Fig. 4.14 and 4.15b. After the time corresponding to this maximum the film begins to

becom t be

explained by the continual overoxidat which resulted in the formation and

release into the solution PANI wi conjugated chains and also in the

forma e solu cts o ad as ne

[1

diminution of the optical den erve ll st s

(e.g. Fig 4). Therefo wing for th we took titative characteristic

the value of the absorbance which corresponds to the ma the kine ve for

the calc ns. Similar to the calculatio e PA-12 stimated action

ization inside the PA-12 film under the same conditions (Fig. 4.15).

First of all, it should be noticed that the bands at 320-340 nm and at 700-750

nm as well as the shoulder at 440 nm presented in the UV-Vis spectrum (Fig. 4.15a)

testify to the formation of PANI in the emeraldine salt oxidation state in the PA-6 film

as in the case of the PA-12 film (Fig. 4.5). But, on the other hand, the rate of the PANI

formation in the PA-6 film is much higher in comparison with that of the PA-12 case

(Fig. 4.15a). Such the difference may be explained by the distinction of the both PM

swelling in the reaction media (Fig. 4.3) and

ant into the PA-6 film and, correspondingly, by the thicker the PANI containing

layer in the PA-6 composite film (Fig. 4.10 and Table 4.2).

The second important observation is the fact that the kinetic curves in the case

of the PA-6 film is completely different from those obtained for the PANI formation in

the PA-12 film (Fig. 4.15). The deceleration of the process in the PA-6 film observed at

∼10 min is rapidly enhanced at 15-20 min (Fig. 4.14). This can be

absorbance. With an increase of the polymerization time the absorbance of the

film decreases and the curve with a well-defined m

e colourless. Such behaviour was also observed by Neoh et al [142] and migh

ion of PANI

of th shorter

f the PANI degr

sity is obs

tion of th

99, 200].

The

ble produ ation, such

d under a

p-benzoquino

udied condition

. 4.1 re, allo is fact as a quan

ximum at tic cur

ulatio ns for th film we e the re

119


Figure 4.14. Kinetic curves of the PANI formation in the PA-6 film at 750 nm at

different (a) aniline (С(NH4)2S2O8 = 0.1 M) and (b) oxidant (Сaniline = 1.33 M)

concentrations:

a - 1 - 0.46 M; 2 – 1.10 M; 3 - 1.33 M; 4 - 1.43 M;

b - 1 - 0.05 M; 2 - 0.07 M; 3 - 0.1 M; 4 - 0.12 M

Polymerization time, min0 20 40 60 80 100 120 140

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Polymerization time, min0 50 100 150 200

Abs

orba

nce

0.0

0.5

1.0

1.5

2.0

2.5

(a) 4

3 2


0 50 100 150 200

Abs

orba

nce

0.000

0.005

0.010

0.015

0.020

0.025

1

1

3

4 (b)

1

2

120


igure 4.15. Differences between the PA matrices, aqueous solution of

1M HCl + 0.1M (NH4)2S2O8 (Сaniline = 1.3 M):

a – UV-Vis spectra of the polymerization process after 4 min

b - Kinetic curves of the PANI formation at 750 nm

400 500 600 700 800 900

Abs

ance

orb

0.0

0.2

0.4

0.6

0.8

1.0PA-6

Wavelength, nm

PA-12

Polymerization time, min0 100 12 160

F

20 40 60 80 0 1400.0

0.5

2.5

PA-6PA-12

1.5

2.0

Abs

orba

nce

1.0

(b)

(a)

121


122

d compared them with those

obtained

n the

s with respect to aniline and oxidant (Fig. 4.16) anorder

for the PA-12 film (Table 4.6).

Table 4.6. Experimentally obtained reaction orders for the aniline polymerization i

PM in the (NH4)2S2O8 water solution in HCl

Polymer film

Reagent

PA-6

PA-12

aniline 2.8 2.8

oxidant (APS) 1.6 1.6

As one can see from the received results the reactions orders with respect to

both aniline and APS are the same irrespective of the PM. Such the result testifies that

the aniline polymerization inside the PA film proceeds according to the same

mechanism.

Differentiation of the kinetic curves of the aniline polymerization in the PA-6

films gives the same parabolic dependencies as for the PA-12 composite films (Fig.

4.17), w

ch the process allows obtaining the conductive PANI containing layer in a

thin sub

technological point of view. Besides, PANI obtained in this way can be doped only by

small-sized inorganic acids and not by organic ones because of the diffusion limitations

hich confirm the autocatalytic character of the aniline polymerization reaction

inside the PM.

The yield of PANI in the PA-6 matrix is much less than that in the PA-12 film

and increases twice at increasing the oxidant concentration – at 0.07 M APS the yield is

17.3% and at 0.12 M APS – 34.4% (Table 4.7). While, the PANI yield stays practically

constant at increasing the aniline concentration to 1.10 M, but during further

augmentation of the aniline content inside the PM to 1.33 M it rises to 28%.

On the whole, the data obtained allow both an easy control of the aniline

polymerization process and the prediction of the PANI yield in the PA matrices. At the

same time it should be noted that the polymerization of aniline inside the PM seems to

be more preferable, when using as a matrix non porous industrial polymers with a poor

solubility. Su

surface layer of the dielectric matrix. Owing to a small thickness (2-5 µm) the

PANI containing layers can quickly react to an external environment changes that

allows applying them in sensors or optical devices.

However, the method of the surface polymerization is not easy from the


Figure 4.16. Double logarithmic plot of the dependencies of the polymerization rate V0

on the concentration of the aniline (a) idant (b) during the aniline

polymerization in the PA-6 film:

1 – 0.1M (NH4)2 2O8 + 1M HCl

2, 3 – (NH4)2S2O8 + 1M HCl (Сanilin 2); Сaniline = 1.10 M (3))

and the ox

S

e = 1.33 M (

lg[AN·104]3.6 3.7 3.8 3.9 4.0 4.1 4.2

lg[V

0·104 ]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.6

1.4

lg[APS·104]2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4

lg[V

0·104 ]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

(b)

1

3

2

(a)

123


(a)

Figure 4.17. The rate of the PANI formation versus the conversion degree of

polymerization reaction in 1 M HCl aqueous solution:

a – at fixed oxidant concentration (С(NH4)2S2O8 = 0.1 M);

b – at fixed aniline concentration (Сaniline = 1.33 M)


Poly

mer

izat

ion

rate

V0·1

04 , mol

/l·m

in

0

200

400

600

800

1000

1200

14000.05 M APS0.07 M APS0.1 M APS0.12 M APS

(b)

Conversion degree, %0 10020 40 60 80

Poly

mer

izat

ion

rate

V0·1

04 , mol

/l·m

in

0.0

0.6

0.9

100.0

200.0

300.0

0.3

0.46 M aniline1.10 M aniline1.33 M aniline(a)

124


Table 4.7. Effect of the oxidant and aniline concentrations on the PANI yield in the

PA-6 film

Oxidant

concentration,

M

Aniline

concentration,

M

General

quantity of

APS in the

solution, mol

General quantity

of aniline in the

PM, mol

Yield of the

PANI

formation, %

0.05 1.33 1.5·10-4 3.39·10-6 0.5

0.07 1.33 2.1·10-4 3.39·10-6 17.3

0.1 1.33 3·10-4 3.39·10-6 28

0.12 1.33 3.6·10-4 3.39·10-6 34.4

0.1 0.46 3·10-4 1.18·10-6 14.2

0.1 1.10 3·10-4 2.82·10-6 15.7

0.1 1.33 3·10-4 3.39·10-6 28

0.1 1.43 3·10-4 3.67·10-6 27.9

of large ganic m cules penetration inside the PM. This problem can be resolved

through formation of the PANI composites when using dispersion polymerization

method [201].

4.5. The aniline polymerization in the dispersed polyamide media

mong the methods described in the literature for the preparation of the

composite materials, the dispersion polymerization allows forming core-shell particles

(where PA-core and PANI-shell). Firstly, this method has the advantage that the host

matrix is hardly changed during the preparation of the conducting composite and,

therefore preserves its desired mechanical properties. The reason for the formation of

the core-shell structure in situ polymerization is that the PANI chains are insoluble in

the medium (water) if their size exceeds a certain value. After this point, the polymer

chains can either nucleate a new particle, aggregate with other polymer chains to form

the PANI structure, or absorb onto an existing surface. If the polymerization of the

monomer is performed in the presence of a host powder matrix, a very large surface

area will be present in the medium for the aniline to absorb onto. If this happens, core-

or ole

A

,

125


shell structures can be formed. The process of the formation of conducting core-shell

composites is schematically depicted in Fig. 4.18.

Figure 4.18. Schematic presentation of the formation of the conducting core-shell

structure via an in situ polymerization

In the dispersion media it is difficult to observe the polymerization process by

UV-Vis spectroscopy. By this reason the progress of the aniline polymerization was

monitored by temperature-potential-pH changes recorded with the special equipment

(see section 2.3.9). Continuous monitoring the redox potential, the temperature and the

pH of the polymerization solution enabled us to follow each stage of the synthesis,

which involved significant changes in the oxidation state and in the protonation state of

the intermediary and final products. The acid-dopants we used are TSA and DBSA,

because they have been reported to obtain rather thermostable PANI [202]. The

obtained profiles are demonstrated in Fig. 4.19. The conditions of the polymerization

process and the concentration of the reagents see in section 2.2.4. As for all other

methods of the PANI formation (electrochemical and chemical in thin subsurface layer

of the PM) we also investigated the aniline polymerization under “free” conditions, i.e.

in the absence of the PA powder (Fig. 4.20). It was made for better understanding the

effect of the PM powder on the aniline polymerization.

rns three distinct periods can be

distinguished (Fig. 4.19a): the induction period, the propagation stage (the period of the

oxidative polymerization) and the period after polymerization [104, 193, 198].

on mixture of aniline, acid and PA

powder no colour changes are observed. During the induction period the temperature of

the reaction mixture does not change. While pH starts to decrease, the value of the

potential begins to increase (Fig. 4.19 and 4.20). The continuous decrease of pH during

the induction period is due to a cont

mixture in a course im 03]. Gradually, the

As one can see, in all obtained patte

Right after APS is added to the reacti

inuous release of TSA or DBSA and H2SO4 into the

of the aniline oxidation and d erization [2

PA particle

aniline NH2HA.

NH2HA.

NH2 HA.

NH2HA.

NH2

HA.

NH2.HA

oxidantPANI shell

PA core

acidic mediumHA

PA particle

aniline NH2HA.

NH2HA.

NH2 HA.

NH2HA.

NH2

HA.

NH2.HA

oxidantPANI shell

PA core

acidic mediumHA

126


Polymerization time, min0 20 40 60 80 100

of different acids:

a – TSA;

b – DBSA

Figure 4.19. Observed during the aniline polymerization time dependence of

temperature-pH-potential in the PA-11 dispersion at room temperature in the presence

Pote

ntia

l, V

pH

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Tem

pera

ture

, 0 C

20

21

22

23

24

25pHPotential, VTemperature, 0C

32.17 min

19.25 min

23.6 0C

Polymerization time, min0 20 40 60 80 100

Pote

ntia

l, V

p

H

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Tem

pera

ture

, 0 C

20

21

22

23

24

25pH Potential, VTemperature, 0C

62.25 min

43 min

23.6 0C

Period after polymerization Induction period

Propagation stage

(b)

(a)

127



Figure 4.20. Time dependence of temperature-pH-pH-potential observed during the aniline

polymerization at room temperature in the presence of different acids:

a – TSA;

b – DBSA

polymerization at room temperature in the presence of different acids:

a – TSA;

b – DBSA

otential observed during the aniline

0 20 40 60 80 100

Pote

ntia

l, V

pH

0.0

0.2

40.

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Tem

pera

ture

, 0 C

pH Potential, V

22

23

25

Temperature, 0C

20

21

24

63.25 min

32.42 min

23.7 0C

(a)


Pote

1.6

1.8

ntia

lH

, V

p

0.0

0.2

0.4

60.

1.

0.8

1.0

1.2

4

Tem

pera

ture

, 0 C

20

21

22

23

24

25pH Potential, VTemperature, 0C

60.25 min

23.7 0C

84.5 min

(b)

128


originally colourless mixture turns blue and later gets darker. The slope of the pH curve

becomes more negative as the polymerization starts. The temperature of the reaction

m

its

m imum (∼23.6 0C (Fig. 4.19 and 4.20)). The deep blue colour observed during the

polymerization at this stage can be assigned to the protonated pernigraniline [192],

namely to its dication diradical form [205]. This fact is confirmed by Stejskal et al

[104]. They performed the aniline polymerization on the glass support and after a

certain period of time being in the reaction media removed this support from the

polymerization solution and measured UV-Vis spectra.

After some time (sufficient for the complete protonated pernigraniline

for ation) which corresponding to maximum of potential on the kinetic curves (Fig.

4.19 and 4.20), the blue reaction mixture starts turning green. During this phase, the

dication diradicals are reduced by a surplus of the aniline [189] to a stable cation radical

structure of the protonated emeraldine [191] and the potential starts to decrease:

Polymerization still proceeds as the aniline is oxidized to PANI by the

pernigraniline dication diradical, until the reduction of pernigraniline to emeraldine is

completed. Protons are released during the polymerization process (scheme (4.5)) and

pH drops even more steeply (Fig. 4.19 and 4.20). After the polymerization has been

completed, the concentration of the protons equilibrates and the pH levels stay without

further changes (Fig. 4.19 and 4.20).

Such a scheme (with the intermediate pernigraniline formation) is confirmed

by the results [206]. The experimental evidence [205] indicated that the intermediate

compound (pernigraniline) is able to act as homogeneous mediator of the electron

t

Thus, the processes involved in

sketched as (4.6).

.

ixture rises steeply to reach a sharp maximum that corresponds to the fact that the

aniline polymerization is the exothermic process [204]. The temperature reaches

ax

m

n

N

H

N. ..n

N

H

N.+ +

HH- e-

+(4.5)

ransfer in redox processes. Mediator decreases the redox potential of the reaction.

the aniline polymerization can be schematically

129


where M is the mediator and M+· is the corresponding cation radical formed in the

course of the oxidation. In this case, mediators are compounds that can easily switch

from the reduced to oxidized form, and vice versa, and that have a redox potential

between the potentials of sulphate/peroxodisulfate and aniline/aniline cation radical

pairs.

Besides, pernigraniline and emeraldine can be converted into each other

according to the scheme (4.6). It is, therefore, not surprising that PANI when formed in

the reaction mixture and promoted the aniline polymerization has an autocatalytic effect

[196

s of

all

reac

he

in

An influence of the acid-dopant nature is also significant - the aniline

polym TSA

than in the pres xplained by the

steric hindrance of the m A. Due

to this, the rate of the an

(4.6)

0.5 S2O82-

SO42-

M

M

NH2

NH2

+.

+.PANI

].

As one can see from Fig. 4.19 and 4.20 profile of the curves almost does not

change in the presence of the PA powder. In contrast to this, the characteristic point

the polymerization process (the potential and temperature peaks, beginning of pH

decrease) are shifted to much less times. Such behaviour testifies to acceleration of

polymerization stages in the dispersion medium. Table 4.8 clearly shows that the

polymerization rate increases in the presence of the PA powder compared to that in the

tion solution without the PA powders. Such effect is observed independently on the

used acid-dopant. This indicates that the polymerization of aniline on the surface of t

PA powder starts earlier than in the whole volume of the reaction media, which is

accordance with observations reported in the literature [104, 192, 198]. So, we may

point out that the polymer powder dispersion has a significant catalytic effect on the

oxidative aniline polymerization for account of the increase of the available surface on

which PANI can deposit.

erization without the PA powder is completed quicker in the presence of

ence of DBSA (Table 4.8). This difference may be e

onomer coupling in a presence of larger anions of DBS

iline polymerization is greater in the case of TSA.

130


T

polymerization pro ig. 4.19 and 4.20)

Acid

S

able 4.8. Parameters of the potential peak position (min) registered during the aniline

cess (from F

Medium TSA DB A

Aniline 63.35 4.5 8

Aniline + dispersed PA 62.232.17 5

In the presence of the dispers wder th cture is ob d – in

case of T the rate of p n i r.

, it should ntioned t the con NI in the osites

was cal ed on the b f the deve method (see section 2.3.1) it was found

that the posites pre with TSA a higher yield of PANI than that in the

presence of DBSA as listed in Table 4.9. Such observation also can be explained by the

steric hi ce in the pr e of DBSA

Estimated practical value

ed PA po e same pi serve

SA olymerizatio s greate

Also be me hat when tent of PA comp

culat asis o loped

com pared had

ndran esenc .

Table 4.9. The yield of the PA-12/PANI bulk composites prepared in the presence of

different acids

TSA DBSA

Theoretical value of

the PANI-acid

content, wt.% content, wt.% yield, % content, wt.% yield, %

1 0.48 48.53 0.61 60.72

2 1.59 79.48 1.13 56.61

4 3.19 79.72 3.61 90.32

6 5.04 83.94

8 7.53 94.14 5.64 70.47

10 9.92 99.22 7.75 77.53

4.6. Conclusions

The study of the chemical aniline polymerization inside the polymer film and

in the polymer powder dispersion enables us to draw the following conclusions:

131


1. The chemical aniline polymerization inside the PM triggers off the interface

of the oxidant solution and the swelled in aniline PM and, thereafter, occurs deep in the

PM. The polymerization process is limited by the matrix’s ability to swell in the oxidant

solution, by the oxidant and monomer diffusion into the reaction zone and, also, by the

presence of the overoxidized PANI in the upper PM layer. All these features result in

the layered structure of the composite films, which is confirmed by Raman

spectrometry. For the first time with the help of Raman spectrometry it has been

established that the thickness of the formed PANI containing layer depends on the PM

structure (2 µm

ability of the matrix to sw

of the reaction solution proves

the exist

diffusion

limitation w

for PA-12 and 4.7 µm for PA-6). This difference is determined by the

ell in the reaction medium.

2. The kinetic peculiarities of the PANI formation in the PA matrices are

connected with the solid state of the PM. Such state of the PM is determined, on one

hand, by the diffusion limitation of the process and, on the other hand, by the

distribution of the monomer (aniline) in a free volume of the amorphous phase of the

semi-crystalline polymer (PA). Also, the ratio of the hydrophilic and hydrophobic

groups in the polymer influences the polymerization process inside the PM. This was

confirmed by the higher rate of the aniline polymerization process in the PA-6 film in

comparison with that in the PA-12 film.

3. The result of the specific character of the matrix polymerization process is

noticeably higher reaction orders with respect to aniline and APS in comparison with

the same process in the solution. At the same time both in the solution and in the PM the

aniline polymerization process runs according to the autocatalytic mechanism and has

three stages – an induction period, a propagation stage and a termination stage. The

determination of the temperature-potential-pH changes

ence of three stages during the dispersed aniline polymerization process.

4. It was established that the aniline polymerization inside the subsurface layer

of the PA matrix proceeds with a rather high yield (∼92 %), in spite of the

hich were confirmed by the spectroelectrochemical measurements. High

efficiency of this process results in obtaining the composite materials with the surface

conductivity of ∼10-4 S/cm. This conductivity level gives hope on an application of the

matrix polymerization process to produce materials with antistatic or sensor properties.

5. The aniline polymerization in the water PA powder dispersion in the

presence of the organic acids (TSA and DBSA) allows obtaining the PANI layer on the

132


surface of the PA particles. As a result the particles with a core-shell structure are

formed.

6. The influence of the acid-dopant on the polymerization process has been

established. It has been found that in the presence of TSA the reaction is completed

quicker than in the presence of DBSA, which is explained by the steric hindrance of the

monomer coupling in the presence of big anions of DBSA.

7. It has been established that the polymerization rate in the presence of the

dispersed PA powder is greater than in the solution of pure aniline. The possible reasons

of this fact can be the catalytic effect of the PA particle surface and the fact that the

polymerization roceeds in the th particle surface. It

is known that the concentration of the reagents in such layers is higher than that in the

solution.

8. It has been shown that the chemical aniline polymerization both in the PM

and in the presence of the PA dispersion takes place according to the same mechanism

as the polymerization of aniline under “free” conditions.

9. Establishing parameters of the aniline polymerization process in the PM and

PA powder dispersion makes it possible to control the properties (conductivity,

thickness of the conducting layer) of such composites.

process p in absorbance layer on the

133

Chapter 5

SPECTROELECTROCHEMICAL,

THERMAL, MECHANICAL,

STRUCTURAL AND RAMAN

PROPERTIES

Chapter 5. Spectroelectrochemical, thermal, mechanical, structural and Raman properties

Introduction

To increase the range of the PANI applications, the composite materials

including non-conducting polymer (PA) and PANI have been obtained by

electrochemical and chemical aniline polymerization (Chapter 3 and 4). In order to

obtain the composites with high technological and physical parameters and properties,

the impact of the nature of the acid-dopants and the type of PA on certain properties of

the composites has been investigated. It is believed that these parameters play an

important role in controlling the properties of the resulting composites. The use of

different methods enables us to compare characteristics of the pure components of the

composite and the composite itself and, thus, to obtain the composites with high

technological and physical properties.

5.1. Spectroelectrochemical investigation of polyaniline and its

composites

5.1.1. Spectroelectrochemical properties of the polyaniline films

The spectral behaviour of the PANI film is determined by the volume charge,

which the PANI film has obtained during electrochemical transformations [120, 207].

One can see this effect clearly from the absorption spectra of the PANI films deposited

onto the SnO2-glass electrode by the cyclic voltammetry method at different applied

potentials in a background electrolyte (Fig. 5.1).

The UV-Vis absorption spectra of the PANI films obtained at different applied

potentials show three absorption bands – at 315, 450 and 875 nm. The position and

absorbance value of these bands changed depending on the applied potentials. When the

applied potential was -0.2 V the PANI film is in completely reduced LE oxidation state

(Fig. 1.1) as expected. In this state PANI has a very low conductivity value [108]. Its

energy structure is similar to the structure of inorganic semiconductors. Absorbance

maximum at 315 nm in this oxidation state (Fig. 5.1, curve 1) is originated from the π -

π* transition of the benzenoid rings [207] and can lead to the electron transitions

between valence and conduction bands. However, a peak at ∼450 nm (cation-radicals)

[22, 108, 207] and a broad peak with the maximum at ∼875 nm [207], which are in

charge of polaron band transitions in the PANI film, in spectrum of LE seems to

134


Figure 5.1. UV-Vis spectra of the PANI film in the background electrolyte (1 М HCl) at

different applied potentials:

1 – Е = -0.2 V; 2 – Е = 0.1 V; 3 – Е = 0.3 V; 4 – Е = 0.5 V; 5 – Е = 0.6 V; 6 – Е = 0.7 V

indicate that a small fraction of oxidized units still remains in the polymer. This may be

due to incomplete reduction of the film or due to LE local reoxidation by oxygen. The

increase of the potential results in the growth of the optical density at 450 and 875 nm,

but the spectra remain similar (Fig. 5.1). With a further increase of the potential to 0.8V,

the charge delocalization on polyaromatic chain occurs, resulting in the absorption peak

shifting to the ∼600-650 nm, at which the imine form of PANI absorbs. In accordance

with [55] this absorption is associated with the bipolaron formation. The LE form is

oxidized to the emeraldine one. Such transitions are accompanied by a smooth visual

colour change of the PANI film: almost colourless at -0.2 V (absorption peak at 330

nm) with potential increasing to 0.7 V the colour changed to green (absorption peak at

875 nm). The oxidation process and the emeraldine salt formation which take place in

the PANI film are in charge of these colour changes. This transition corresponds to the

first couple of the redox peaks on the cyclic voltammogram (Fig. 3.1a), that well

confirm reversible changes, which take place in the PANI film.

1

2

7 65

4

3

Wavelength, nm300 400 500 600 700 800 900 1000

Abs

orba

nce

0.0

0.1

0.2

0.3

0.4

0.5

6 5 4 3

2

1

135


By analyzing the absorption spectra of the PANI films obtained under

potentiostatic conditions one can see that the films formed at 0.7 and 0.75 V (Fig. 5.2a,

curves 2 and 3) have the highest absorption at ∼650-700 nm (i.e. in the region which is

typical of the conducting PANI in the emeraldine form). As it was discussed before, this

fact testifies to the formation of bipolarons [119, 208]. The UV-Vis spectrum of PANI

formed at 0.6 V (Fig. 5.2a, curve 1) does not reveal any maximums, which testify to the

formation of PANI with a shorter conjugation length and in a non-oxidized state.

Moreover, the quantity of PANI formed at 0.6 V is very small that well agrees with

electrochemical results (see chapter 3.1.2). On the contrary, at the potential 0.8 V (Fig.

5.2a, curve 4) the deep oxidation and overoxidation take place which give rise to

decreasing the optical intensity of the peaks. Under these conditions we received the

dark blue PANI film (pernigraniline). The shape of the spectrum testifies to the polymer

degradation, which results in an irreversibility of the PANI oxidation process. Thus, in

order to obtain the conducting non-overoxidized PANI film its formation should be

performed at potentials between 0.6 V and 0.8 V.

The spectra of the PANI films formed in galvanostatic mode show similar

behaviour: at і = 0.001 mA/сm2 a yellow film of PANI in the non-oxidized state is

formed (Fig. 5.2b, curve 3); at і = 0.1 mА/сm2 a non-homogeneous overoxidized PANI

film is obtained (Fig. 5.2b, curve 1) and at the intermediate і = 0.005-0.01 mА/сm2

rather homogeneous electroactive green PANI films are formed (Fig. 5.2b, curves 2 and

3).

Obtained results are in a good agreement with the previously discussed data

(Fig. 3.1, 3.5, 3.6) and with the literature [108, 119]. The colour changes in the PANI

films are evidence of the existence of the several oxidation states which differ in the

oxidation levels. Such spectral PANI properties can be used in different colour

electrochromic displays or sensors, etc.

5.1.2. Spectroelectrochemical properties of the conducting composite films

The results of spectroelectrochemical measurements have shown that in the

case of the aniline electrochemical polymerization in the PM two absorption maxima –

at ∼440 nm and at ∼825 nm are observed (Fig. 5.3) like in the case of the pure PANI

film (Fig. 5.1). These maxima, in accordance with the literature [108, 120, 207], are

connected with the polaron (localized cation-radical) formation. It should be pointed out

136


Figure 5.2. UV-Vis spectra of the PANI films obtained under (a) potentiostatic and (b)

galvanostatic conditions in the aqueous solution of 1М HCl, aniline concentration 0.5 М:

a - 1 – Е = 0.6 V; 2 – Е = 0.7 V; 3 – Е = 0.75 V; 4 – Е = 0.8 V;

b - 1 – i = 0.001 mA/cm2; 2 – i = 0.005 mA/cm2; 3 – i = 0.01 mA/cm2;

4 – i = 0.1 mA/cm2

Wavelength, nm400 600 800 1000

Abs

orba

nce

0.0

0.1

0.2

0.3

0.4

0.5

0.6


Abs

orba

nce

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

(a)

3

4

2

1

(b)

1

3

2

4

137


Figure 5.3. UV-Vis spectra of the composite PM/PANI films at different applied

potentials:

1 – PVA, E= -0.2 V; 2 – PVA, Е = 0.5 V;

3 – PA-12, Е = -0.2 V; 4 – PA-12, Е = 0.5 V

that on the spectrum measured at -0.2 V the peak at 825 nm is also observed as in the

case of the PANI film (Fig. 5.1). This fact can be explained by the incomplete reduction

of the film. During the oxidation from -0.2 V to 0.5 V the peaks shifted from 825 nm to

770 nm and from 440 nm to 420 nm (Fig. 5.3). Such a hypsochromic shift resulted in

the colour change of the film from pale yellow to green which is similar to the

behaviour of pure PANI. Also, it should be noted that the absorption intensity of PANI

formed in the PA-12 matrix is higher than that in the PVA matrix (Fig. 5.3, curves 2 and

4) that can be explained by the different PANI amount obtained in the PM. This

observation is also in a good agreement with the obtained dependencies of the first

anodic peak on the bare and covered with the PM electrodes (Fig. 3.13). Such a

difference in the PANI quantity in the different PM can be explained by the

physicochemical interaction between the PANI molecules and amide groups of the PA

matrix – the formation of the hydrogen bonds.


Abs

orba

nce

0.0

0.1

0.2

0.3

0.4

1

4

2

3

138


One can suppose that during the aniline polymerization in the PM the presence

of this interaction can lead to a certain orientation of aniline molecules. This orientation,

in its turn, can effect the conjugation length and the doping level of PANI obtained on

the bare and covered with the PM electrode surfaces. Indeed, the peak in the UV-Vis

spectra at 770 nm, which was attributed to polaron transition in PANI, was broader and

shifted to the shorter wavelength (Fig. 5.3, curves 2 and 4) in comparison with those for

PANI formed on a bare electrode (Fig. 5.1, curve 4). This fact allows us supposing a

shorter conjugation length of PANI formed inside the PM. However, judging by the

shifting value the PANI conjugation length in both matrices is practically the same and

doesn’t depend on the matrix structure. In spite of this, the structure of the PM is

revealed in a different doping level of the formed PANI structures. Thus, according to

the literature results, one can judge about this level by the ratio of the absorption

intensity of quinonediimine structures (400-500 nm) and polarons presented in the

polymer (800 nm). Having analyzed the received UV-Vis spectra, it is easy to see that

in the case of pure PANI the polaron absorption dominated (Fig. 5.1 and 5.2). On the

contrary, on the UV-Vis spectra of the composite films the absorption of the

quinonediimine structure is revealed (Fig. 5.3). The quantity of these structures

increases from PA-12/PANI to PVA/PANI films. In other words, the PVA matrix

makes the doping process more difficult despite of its less resistance, in comparison

with the PA-12 matrix. The reason of such difficulty may be a stronger interaction of

PANI with the matrix. Such explanation is in good agreement with the voltammetry

results of the potential shift of the PANI peaks in both PM (Fig. 3.14).

The changes of the UV-Vis spectra of PANI can be caused not only by the

different reaction conditions, but also by the arbitrary oxidation under the influence of

the external factors. During the discharge of the PANI films at -0.2 V the films become

colourless, but after being in the air during some period of time (< 3 min) their green

colour is gradually restored. At further potential applying of -0.2 V the oxidized PANI

state is rather easily reduced to the primary state, however, in the air the process of the

colour changing is repeated. This fact can be explained by the oxidation of the PANI

reduced film (leucoemeraldine state) by the oxygen which is in the air.

139


5.2. Thermal behaviour of polyaniline and its composite materials

For all practical purposes, the stability of the conducting composites is very

important aspect to pay attention to. The thermal stability and thermal behaviour of

PANI are the subjects of many investigations [75, 202, 209]. However, since the

synthesized composite materials are based on PANI and PA, their thermal stability can

be different from that of the neat polymers. To evaluate the possible effect of PANI in

this case, the thermal stability of the polymers and the composites was investigated by

TGA.

5.2.1. Thermal behaviour of polyaniline

The thermal gravimetric analysis results for the dedoped and doped PANI

powders are given in Fig. 5.4. A weak weight loss step between 60 and 100 0C on the

weight curve of PANI-base is attributed to the losses of small molecules such as

solvents and impurities [210]. A strong weight loss step, which commences at about 350 0C, is attributed to the decomposition of the PANI backbone involving chain scission

and fragmentation [211].

In contrast to PANI-base, the thermograms of doped PANI (Fig. 5.4) show

three-step weight loss process, which is similar to the observations done by Ding et al

[75] and Wang et al [209]. The small fractions of weight loss (∆m = 2-7 %) below 120 0C may be attributed to the loss of moisture in the PANI samples in accordance with

[75]. The amount of the weight loss due to the evaporation of moisture is greater for the

PANI-HCl and PANI-H2SO4 powders than for the PANI-TSA, PANI-DBSA, PANI-

CSA and for the dedoped PANI powders. This indicates, obviously, that the HCl- and

H2SO4-doped PANI absorbs water easier than other samples. These TGA results are in

good agreement with Matveeva et al results [212], who reported that it is very difficult

to remove completely the bound water by drying. The second weight loss, in the range

of 120-250 0C, is attributed to the dedoping process of the PANI powder [209] and the

last one is due to the decomposition of the PANI chains [211]. This occurs at 350 0C,

which is consistent with the previous results [209] and with the TGA trace for dedoped

PANI.

140


Figure 5.4. TGA traces for the chemically synthesized PANI powder, 10 0C/min

rom Fig. 5.4 and Table 5.1 we can see that the weight loss of the PANI-acid

powders in the temperature range of 180-280 0C is about 5-18 % depending on the acid-

dopant and it is due to the evolution of the free dopant [75]. The heavy weight loss at

about 370 0C is corresponding to the degradation of organic acid (Fig. 5.5). Besides, a

higher thermal stability of DBSA-doped PANI as compared to CSA-doped PANI (Table

5.1 and Fig. 5.4) agrees with Han et al results [213] and may be caused by the different

evaporation temperatures because the PANI-CSA powder contains the largest content of

moisture due to the hydroscopic nature of CSA.

rom the analysis of the obtained results it is easy to see that the PANI

powders doped by the different acids can be disposed by the thermal stability in the next

row:

HCl < H2SO4 < CSA < TSA < DBSA.

ake a conclusion that the nature of the dopant anion plays an

important ro

Temperature, 0C

0 200 400 600 800 1000

Wei

ght,

%

0

20

40

60

80

100dedoped PANIPANI-HClPANI-H2SO4

PANI-TSAPANI-DBSAPANI-CSA

F

F

So, we can m

le in the thermal stability properties of PANI.

141


Table 5.1. Temperature values for the different weight losses for PANI

Acid-dopant T∆m=10%, 0C T∆m=20%, 0C T∆m=30%, 0C T∆m=40%, 0C T∆m=60%, 0C

HCl 144 297 405 472 601

H2SO4 132 296 350 402 498

TSA 272 321 389 443 551

DBSA 270 302 382 453 554

CSA 216 267 310 404 513

Dedoped 428 480 521 559 630

Figure 5.5. TGA traces for the acids used as dopants, 10 0C/min

5.2.2. Thermal behaviour of the conducting composite powders

Preliminarily we investigated the thermal behaviour of the pure PA-12 powder.

It was established that PA-12 degraded at temperature of about 400 0C (Fig. 5.6), which

corresponded to degradation of the PA chain [214].

The thermal patterns of the synthesized composite PA-12/PANI powders

doped with different acids are also displayed in Fig. 5.6. The composite materials show

Temperature, 0C0 200 400 600 800 1000

Wei

ght,

%

0

20

40

60

80

100TSADBSACSA

142


m

he

e loss is

erature up to

posite

powders sim perature

higher than ∼410 C is, probably, due to the complete degradation of the matrix polymer

and PANI backbones after the loss of the dopants [202].

CSA) at the

temperat

odriguez and Akcelrud in the

polyuret

Temperature, 0C0 200 400 600 800 1000

Wei

ght,

%

0

20

100PA-12PA-12/PANI-HClPA-12/P 2SO4ANI-HPA-12/PA TSA

40

60

80 NI-PA-12/P BSAANI-DPA-12/P SAANI-CPA-12/PA dedoped

Figure 5.6. TGA traces for the composite PA-12/PANI powders, 10 0C/min

slower polymer decomposition compared with that of the PANI powders (Fig. 5.4). The

ain feature of the TGA traces of the PA-12/PANI powders is the presence of only two

weight loss stages for all the acid-dopants with the exception of CSA (Fig. 5.6). T

TGA trace of CSA presents three decomposition stages (the additional stag

observed at the temperature from 300 0C to ∼380 0C). This may be attributed to a lower

thermal stability of CSA (Fig. 5.5) and its hydroscopic nature.

The first stage weight loss starting practically from the room temp

110 0C corresponds to a loss of water molecules/moisture presenting in the com

ilar to pure PANI samples (Fig. 5.4). The weight loss at the tem0

The minor weight loss for the composites (except PA-12/PANI-

ure up to 400 0C and the great one above 400 0C (Fig. 5.6) indicates that these

composite materials have a good thermal stability close to that of pure PA-12. The

similar thermal behaviour was observed by R

hane/PANI based composites [215].

NI-

143


However, it should be also pointed out the existence of a significant difference

in TGA traces of the PA-12/PANI samples doped by organic and inorganic acids (Fig.

5.6). Such behaviour can be explained by the fact that inorganic acids (HCl, H2SO

rmed more stable salts with PANI.

It was found that the PANI content in the composite is only 2.38 wt.% on the

basis of the dedoped PANI powder (∼3-5 wt.% in the doped state). Therefore,

weight loss before 400 0C, which could be ascribed to the decomposition of the PANI

backbone, is relatively small.

On the whole, the obtained thermograms of the PA-12/PANI powder

composites powders testified to the higher thermal stability of PANI in the com

aterial (compare Fig. 5.4 and 5.6 and Tables 5.1 and 5.2).

Table 5.2. Temperature values for the different weight losses for the PA-12/PANI

composites

Composition of the

4) are

fo

the

posite

m

T∆m=10%, T∆m=20%, T∆m=30%, T∆m=40%, T∆m=60%,

C sample 0C 0C 0C 0C 0

pure PA-12 438 453 460 466 474

PA-12/PANI-HCl 440 455 464 469 478

PA-12/PANI-H2SO4 448 462 469 473 481

PA-12/PANI-TSA 408 420 428 433 446

PA-12/PANI-DBSA 401 415 421 428 440

PA-12/PANI-CSA 371 430 452 463 478

PA-12/PANI-dedoped 451 463 468 474 483

An enhancement of the thermal stability of the PA-12/PANI composites is

clearly indicated by a temperature rise to about 100 0C in comparison with the pure

PANI powders (the main weight loss is observed at ∼420 0C, while for pure PANI it is

observed at ∼300 0C). Also, the increase of the thermal stability indicated some kind of

interacti

.

on between PANI and PA-12. A similar behaviour has been observed by Das et

al [216] for polyacrylamide/PANI blends and by Rodrigues et al [217] when studying

the lignin/PANI blends through TGA. They suggested the formation of the hydrogen

bonds between two polymers, which provides an increase of the thermal stability of the

composite comparing to the pure components

144


In order to study an influence of the PANI content on the thermal stability of

the composite materials we prepared and investigated with the help of TGA the

composites with a different PANI content and doped by different acids (Fig. 5.7, Table

5.3). As one can see from obtained thermogramms, for all studied composites two

stages of

PANI content, increasing with increasing the PANI

concentr

NI fraction.

C

sa

T∆

0C

T0C

T∆

0C

T∆

0C

T0

the weight loss are observed – a minor weight loss at 80–100 0C (inset in Fig.

5.7) and the second one – at 380-420 0C. On the whole, the thermal stability of the pure

PA-12 powder is maintained irrespective of the PANI content. The mass loss at above

380 0C is determined by the

ation, probably, for account of the dopant evolution (Table 5.3). This

observation also confirmed the above stated supposition about increasing of the thermal

stability of the composites by the presence of the dedoped PA

Table 5.3. Temperature values for the different weight loss for the PA-12/PANI

composites

omposition of the

mple m=10%, ∆m=20%,

m=30%,

m=40%,

∆m=60%,

C

pure PA-12 434 452 459 467 477

PA-12/PANI-TSA

(0.48 wt.%)

43 45 46 46 49

1

0

3

73

PA-12/PANI-

DBSA (0.61 wt.%)

41 44 45 46 49

5

6

3

74

PA-12/PANI-TSA

(7.53 wt.%)

40 41 42 42 40

5

2

7

42

PA-12/PANI-

DBSA (

7.75 wt.%) 382 406 416 423 439

As one can see from Table 5.3, the composite materials doped by TSA display

a little bit higher thermal stability irrespectively of the PANI content than those doped

by DBSA. Such behaviour may be explained by different portions of PANI in the

PANI-acid complex. On the other hand, with the rise of the weight loss the difference

becomes less significant due to the dedoping process, i.e. because of the removal of the

acid molecules from the composite powder. This means that at a temperature higher

than that characteristic of the dopant complete removal, the composite material contains

only dedoped PANI which, as was shown before (Fig. 5.4 and Table 5.1), degrades at

145


0 200 40

Figure 5.7. TGA traces for the composite PA-12/PANI powders doped by (a) TSA and

(b) DBSA with different PANI-acid content, 10 0C/min

0 600 800 1000

Wei

ght,

%

0

20

40

60

80

100

PA120.48 wt.% 1.59 wt.% 3.19 wt.% 5.04 wt.%7.53 wt.% 9.92 wt.%

20 30 40 50 60 70 80 90 100 110 120 130

Wei

ght,

%

97

98

99

100

101

Temperature, 0C

(a)

T eremp atur 0Ce,

(a)

(b)

Temperature, 0C0 200 400 600 800 1000

Wei

ght,

%

40

60

80

100

0

20

PA120.61 t.% w1.1 3 wt.%3.61 t.% w5.64 t.% w7.7 5 wt.%

Temperature, 0C20 30 40 50 60 70 80 90 100 110 120 130

Wei

ght,

%

97

98

99

101

100

(b)

146


higher temperatures in comparison with the doped samples.

Also, it is clearly seen that with the increasing of the PANI content in the

composite material the temperature, at which the beginning of the significant weight

loss occurs, is decreased. This feature is preserved independently of the used acid-

dopant (compare 438.75 0C and 399.67 0C for PANI-TSA with 419.23 0C and 381.86 0C

for PANI-DBSA for the ∼0.5 wt.% and ∼7.5 wt.%, respectively).

On the whole, one can conclude that the thermal stability of the composite

materials is close to that of pure PA and the nature of the acid-dopant was found to

influence the thermal stability of the whole composite. Also, it should be pointed out

that the temperature of the significant weight loss of the composite because of the

dopant removal and polymer degradation is significantly higher than that of the

composite processing. The obtained data testify that it is possible to produce the

conductive polymer blends with satisfactory thermal properties by the aniline

polymerization in the PA water dispersion.

5.3. Mechanical investigation of the conducting composite films

As it was already mentioned (see section 1.1.3) the main disadvantage of

PANI, which often hinders its practical applications, is its poor mechanical properties.

Up to now some efforts to improve these properties have been done. Among them is the

preparation of the PANI films of the gel state [82] or orientation of the PANI film [83].

Also, it was found that the crosslinking of the PANI films made of the concentrated (8

wt.%) NMP solutions significantly increased its tensile strength [81]. The dopant and

moisture content have a great influence on the mechanical properties of the films, as

well [82].

In spite of all these data, the preparation of conducting composite materials

based on PA and PANI seems to be more preferable from the technological point of

view for an improvement of the mechanical properties of the films. Polyamide itself can

form very good quality films of appreciable strength [190].

The mechanical properties of the PA-12/PANI films, pressed at 195 0C of the

c

and the results a

orresponding powders (detailed description see in the section 2.2.2), were measured

re shown in Table 5.4.

147


In the case of the pure PA-12 film the tensile strength is 52.8 MPa and for the

dedoped

sion is confirmed by

the resul

the PA-12/PANI composite films

Composition of the sampl Tensile strength, MPa

PA-12/PANI composite film this value is practically the same (51.4 MPa).

However, for the doped PA-12/PANI films the tensile strength of the films decreases.

It is known [5, 215, 216, 218, 219] that under the same conditions of

preparation of the PANI based composite materials the nature of the acid-dopant is a

factor, which determines properties of the composites. This conclu

ts of the mechanical measurements (Table 5.4). In the case of the doping by the

inorganic acids (HCl, H2SO4) the tensile strength decreases from 52.8 MPa to ∼33 MPa.

However, when organic dopants (TSA, DBSA) were used as a doping agent, composite

films with a tensile strength of ∼43 MPa could be fabricated. These data indicate that

the strength of the PANI films could be increased by blending with the common

polymers as a matrix and, besides, by using certain acid-dopants.

Table 5.4. Mechanical properties of

e

pure PA-12 52.8

PA-12/PANI dedoped 51.4

PA-12/PANI-HCl 33

PA-12/PANI-H2SO4 35

PA-12/PANI-TSA 43

PA-12/PANI-DBSA 47

The higher tensile strength in the case of organic acid-dopants used may be

explained by the known fact that organic acids perform triple role – of a dopant, a

plasticizer [220] and a compatibilizer. Besides, the composite films doped by such

function

in Fig. 5.8 as a function of

the PAN

se of the mechanical properties of composites

independently on the acid used.

alized acids when producing the film by the compression molding technique

preserve a higher conductivity value as it will be shown later (see Chapter 6).

The mechanical properties (tensile strength) of the pure PA-12 and the PA-

12/PANI-TSA and PA-12/PANI-DBSA films are presented

I-acid content. As one can see, the tensile strength of the composite films at

rather low PANI contents is not far from that of the pure PA-12 film. But the increase of

the PANI content leads to the decrea

148


It should be noted that in the case of DBSA, the tensile strength is not

significantly affected by the addition of PANI-DBSA up to 4 wt.%. However, the

presence of more than ∼4 wt.% of PANI-DBSA in the films leads to a decrease in the

value of

Figure 5.8. The tensile strength of the PA-12/

reasing the PANI content, the PANI component

the tensile strength. At the same time in the case of TSA-doped PANI, with

increasing the content of the conducting PANI-TSA complex the mechanical strength of

the composite film at once decreases. The tensile strength decrease testifies, probably,

that the PANI-acid complex in the composites behaves as a defect in the PM. Thus, at

low PANI-acid complex content, a good and homogeneous dispersion is obtained. A

higher PANI-acid content and the using TSA as an acid-dopant may cause great defects

in the matrix and the mechanical perturbations, which will decrease the tensile strength

(Fig. 5.8).

PANI-acid films with different conductive

complex content

The observed trend is in some agreement with previous results [219, 221, 222]

obtained for the conductive composite materials based on other common polymers.

Small amount of PANI disturbs morphology of the PA matrix itself, thereby, lowering

the tenacity value. When inc

PANI-acid content, wt.%0 2 4 6 8 10

Tens

ile s

treng

th, M

Pa

0

10

20

30

40

50

60 TSADBSA

149


increasin

owder. It is known that polyamides are subject to the acidic hydrolysis

[158]. So, additional amount of acid can lead to such (acidic) kind of hydrolysis

resulting in a decrease of the tensile strength of the film.

On the other hand, the rise in the pressing temperature from 195 0C to 240 0C

(Fig. 5.9b) also results in the decrease of the tensile strength. This may be, probably,

explained by the fact that with the rise in the pressing temperature the dissociation of

the PANI-acid complex increased and, as a result, the certain amount of the “free” acid

appeared in the composite. This acid acts as in the case of the additional doping, i.e.

leads to the hydrolysis of PA.

Also, the pressing of the additional doped powders at higher temperatures leads

to the same result and, correspondingly, the value of the tensile strength decreases.

Therefore, the obtained results allow confirming that the tensile strength of the

composite materials based on PA and PANI can be control by changing the nature of

the acid-dopant, the content of the conducting PANI complex and the pressing

temperature. The composite films with sufficient tensile strength value could be

obtained.

5.4. Structural studies of the surface conducting composite films

Chapter 4, the feasibility of the surface PA/PANI composites synthesis was demonstrated and the kinetic peculiarities of this process were estimated. Besides, the

influence of the PM structure on the thickness of the conducting layer was shown.

owever, we may suppose that the structure of the PM may influence not only

the thickness of the PANI containing layer, but also the distribution of PANI inside the

gly contributes to the strength of the system and tenacity somewhat decreases.

Previously Gangopadhyay et al [219] reported a similar character of the mechanical

properties of the PVA/PANI composites.

The influence of an additional doping, i.e. an additional treatment in TSA

solution (see section 2.2.4), and the pressing temperature on the mechanical properties

of the composite materials with the different PANI-TSA content has also been

investigated. The results of the measurements are depicted in Fig. 5.9. It has been

established that the additional doping by TSA leads to a decrease of the tensile strength

of the composite films (Fig. 5.9a). Such influence may be explained by the fact that

during the doping process an additional quantity of the acid is introduced inside the

composite p

In

H

150

Chapter 5. S

151

b – influence of the pressing temperature

pectroelectrochemical, thermal, mechanical, structural and Raman properties

Figure 5.9. Tensile strength of the PA-12/PANI-TSA composites as a function of the

PANI-TSA content:

a – influence of the additional doping;

PANI-TSA content, wt.%0 2 4 6 8 10

0

10

30

40

50

60

a M

Png

th,

stre

ileTe

ns 20

without additional dopingwith additional doping(a)

PANI-TSA content, wt.%0 2 4 6 8 10

Tens

ile st

reng

th, M

Pa

0

10

20

30

40

50

60195 0C220 0C 240 0C

(b)


PM layer and the final composite structure. The concernment of this fact is very

important because the conductive properties of the composites depend on molecular

distribution of the conductive clusters with respect to the PM [223, 224]. By these

reasons the investigation of the morphology of the composite films was performed. The

[147] for the PA-6/PANI

The difference between the PM is clearly seen from the optical pictures of

surface c

ANI

ob

PANI formed in the PA-6 film appears as a very homogeneous film with a continuous

network structure (Fig. 5.11b), whereas the PET/PANI film presents a granular structure

(Fig. 5.11c).

Two phases are clearly seen for the PET/PANI composite (Fig. 5.11c): the neat

PET, characterized by the fracture surface and PANI, characterized by aggregates and

agglomerates organization. It has been shown by standard AFM measurements [223]

that the doping of the composite films based on PANI and a PM such as PET drastically

modifies the film surface topography showing a re-organisation of PET and PANI

flexible chains under the influence of the acid. It has been found that the pure PET and

the dedoped form of the composite PET/PANI film exhibit comparatively flat surfaces,

whereas doping leads to the appearance of the hilly features. Also, it has been found that

not only doping (i.e. appearance of charge carriers on the PANI chains), but the acid-

dopant also influences the surface morphology of the composite films [223]. In

particular, it has been found that HClO4 molecules induce much stronger changes in

comparison with the HCl case. Specifically, it is possible to believe that this effect is

caused by a new packing of the amorphous part of the PET and PANI which is induced

composite film based on PET was also investigated in order to determine the influence

of the PM structure.

The optical microphotographs for HCl-doped PA-12/PANI composite film

show that the pure PA-12 film has uniform surface (Fig. 5.10a) in comparison to doped

composite films (Fig. 5.10b). Similar distribution of PANI was found by Zhang et al

[225] for the PA-11/PANI composites and by Basheer et al

and PA-12/PANI composites with the help of TEM.

omposites (Fig. 5.11). One can see from this figure that the structure of the PM

is important in determining the distribution and the organisation of the conducting

clusters in the synthesized composites. As it can be observed, the distribution of P

tained in the PA-11 film (Fig. 5.11a) is similar to that in the PA-12 film, probably,

due to the similarity of the molecular structures of the PM (Fig. 2.1). On the other hand,

152


Figure 5.10. Optical micrographs of the pure PA-12 film

PA-12/PANI-HCl film (b)

(a) and of the composite

PA-6/PANI-HC HCl (c) films

(a) (b)

(c)

(a) (b)

Figure 5.11. Optical micrographs of the composite PA-11/PANI-HCl (a),

l (b) and PET/PANI-

153


by Cl- and ClO4- anions penetration. Obviously, larger ClO4

- anions, when penetrating

into the PM, deform it stronger than Cl- ones [223].

In order to confirm the influence of the doping process on the topographical

features of the composite polymer film, we performed AFM measurements for the

composite PA-6/PANI-HCl film (Fig. 5.12a). As one can see from the obtained image,

the surface deformation is also observed. But, in contrast with the PET/PANI composite

[223, 22

ing layer reveals a distribution of

conducti

4], for the PA-6/PANI composite film a rather uniform distribution of PANI in

the surface layer of the film is demonstrated (Fig. 5.12a).

The change of the surface morphology is accompanied by the modification of

the electrical properties of the film surface. As one can see, the electrical image (Fig.

5.12b) performed on the surface of the PANI contain

ng clusters with a resistance value around 105 Ω, dispersed in the PM having a

high resistance value (over 1011 Ω). It is evident that these clusters are different by

shape and irregularly distributed in the PANI containing layer, that leads to a hilly shape

of the surface layer.

Figure 5.12. Topographical (a) and electrical (b) AFM images of the surface PA-

6/PANI-HCl composite film obtained with the “Resiscope” in contact mode

(a) (b)

154


In the case of the PET matrix (Fig. 5.13), the conducting clusters are rather

dispersed and no clear evidence of percolation is observed. On the contrary, for the

sample based on the PA-6 film, the conductive ilm

surface (Fig. 5.12b). It has been shown [143] th t the percolation threshold in the latter

case reaches for only 4 wt.% of PANI. As a result, the electrical macroscopic properties

are different: the film surface resistance measured at room temperature by a standard

four-electrode technique is found to be 20 kΩ and 4 MΩ for PA-6 and PET matrices,

respectively. This difference cannot be attributed to the initial concentration of aniline

in the swelled film and, correspondingly, to the PANI content. The samples being

pre t

of aniline (around 12 wt.%). I etry (chapter 4 and

[226]) that the thickness of the PANI containing layer was higher in the PA-6 matrix

(4 m) than in the PET film (∼2 µm). The structure of the PM may, probably, explain

the encountered differences. The found struct posite films have been

correlated with the observed electrical conductivity values (Chapter 6).

Figure 5.13. Topographical (a) and electrical (b) AFM images of the surface

PET/PANI-HCl com 223]

As a who estify to a phase

segregation revealed by the presence of the clusters of various conductivity values in the

thin subsurface layer of the composite film.

clusters occupy a large part of the f

a

pared in the same way, in both cases the pure films took up roughly the same amoun

t was also shown by Raman spectrom

.7 µ

ures of the com

posite film obtained with the “Resiscope” in contact mode [

le, the AFM and the optical microscopy data t

(a) (b)

155


5.5. Raman spectrometry measurements

1.1 and 1.2) [227, 228]. Most

of them

he fact that a lot of articles describe the Raman behaviour of pure

PANI in different oxidation states, only a few Raman studies give the characterization

of the PANI composite materials. example, Pereira da Silva et al [231] reported on

the evolution of the Raman spectra of PMMA/PANI composite films prepared by

mixing solutions of PANI-CSA and PMMA using m-cresol as a solvent. The blue shift

of the frequencies of the carbonyl band was observed and the authors correlated it with

the indication of the chemical interaction between PANI and PMMA.

In our study, we used Raman spectrometry in order to identify the presence of

PANI and its forms in the obtained composites and, also, in order to determine the

discrepancies between the composite materials with different PANI contents.

5.5.1. Raman spectrometry investigation of polyaniline

For several years, PANI was extensively studied by Raman spectrometry.

However, because of the PANI structure complexity, Raman studies remained

essentially limited to the qualitative discrimination between the different forms of

PANI. Numerous works were done in order to identify the different vibrations

characteristic of the different PANI oxidation sates (Fig.

dealt with the study of the PANI insulating forms: leucoemeraldine, emeraldine

and pernigraniline bases. More recently, several papers reported on the interpretation of

the Raman spectra of the PANI conducting form (ES) [229-231]. It was found a

correlation between the appearance of a new band around 1330 cm-1 and the PANI

protonation process. The authors [229, 230] assigned this band to the stretching of >C-

N+· bonds.

In spite of t

For

Typical Raman spectra obtained for the pure PANI powder in different

oxidation states are shown in Fig. 5.14. The positions of the main vibrations were

compared with the typical vibrations of ES and EB reported in the literature under

sim ed

(Fig. ring

stretching vibration of the benzenoid ring at 1622 cm-1 [232] shifts to 1604 cm-1 after

the dedoping process. The C=C ring stretching vibrations of the quinoid rings at 1591

ilar experimental conditions. A comparison of the spectra measured for the dedop

5.14, curve 1) and doped (Fig. 5.14, curve 2) powders reveals that the C-C

156


Figure 5.14. Raman spectra of the PANI powders in different oxidation states

(λ = 514.5 nm, t = 300 s, p = 0.04 mW):

1 – dedoped PANI (EB) powder;

2 – doped PANI-HCl powder;

3 – doped PANI-H2SO4 powder

cm-1 [230], which cannot be observed in the PANI-HCl sample, grow in intensity when

more quinoid units are formed during the dedoping process. At the same time, the C-H

in-plane bending deformation of the benzenoid rings at 1191 cm-1 [229] completely

disappeared from the Raman spectrum of dedoped PANI and a new band assigned to C-

H in-plane bending deformation of the quinoid rings at 1165 cm-1 appears for the

dedop f the

d

cm-1. They are assigned, respectively, to the C-N stretching vibrations of the benzenoid

units [44

ig. 1.2). Lindfors et al [234] have studied

2

1

3

Wavenumber, cm

ed PANI powder. Also, two new peaks appear in the Raman spectrum o

edoped PANI sample: the one at 1221 cm-1 and the broad band around 1488 - 1557

] and the C=N stretching vibrations of the quinoid units [233]. The presence of

these bands are consistent with the structure of the EB form of PANI inasmuch as it

consists of both >C=N- and >C-NH- units (F

-11000 1200 1400 1600 1800

Ram

an in

tens

ity, a

.u.

0

5000

10000

15000

20000

25000

30000

35000

1622

1191

1591

1604

1165

1221

1488

1258 13

2013

40

1557

1409

1191

1258 13

40 1622

1165

1

2

3

157


the ES-EB transition of the PANI films in solutions with different pH values. They

fou hen pH is increased. And that

simultaneously, the C-N+· stretching vibrations of the semiquinone radicals at 1258 cm-1

[228] dis

internal redox process associated with this type of protonation, results in

lowering

transition can be

induced

e Raman

spectrum ig. 5.14, curves 2 and 3). As one may see

in the istic bands of doped

PANI ar

nd that the band at 1488 cm-1 grows strongly w

appear almost completely (Fig. 5.14, curve 1).

In the Raman spectrum of the dedoped PANI form the C-C stretching

vibrations at 1557 cm-1, which is probably related to the quinoid units [227], as well as

the C-C stretching vibrations of the quinoid units at 1409 cm-1, are also observed.

In all cases, the ES form can be undoubtedly identified when a band around

1320-1340 cm-1, assigned to protonated nitrogen [229], dominates the spectrum (Fig.

5.14, curve 2). Indeed, it should be pointed out that none of the insulating forms of the

PANI (LE, EB and PNA) gives rise to vibrational modes in the 1300-1400 cm-1 spectral

region [228, 230]. The presence of the peaks in this region is, therefore, inherently

associated with the protonation process by means of the radical formation mechanism.

The

the order of the bond between C and N, which now becomes intermediate

between the amine (>C-NH-) and the imine (>C=N-) bonds [6].

It should be noted that the characteristic >C-N+· stretching vibrations of the

delocalized polaron charge carriers at 1320-1340 cm-1 can still be seen in the spectra of

dedoped PANI. This feature may be explained by the minor fractions of the ES form,

which are, probably, still present in the dedoped sample. On the other hand, as it was

established by Lindfors and Ivaska [234] the benzenoid to quinoid

by the incoming laser light resulting in the C-N+· stretching vibration observed

in the spectrum (Fig. 5.14, curve 1).

We also investigated the influence of the acid-dopant nature on th

of PANI in the ES oxidation state (F

Raman spectrum of H2SO4-doped PANI, all the character

e preserved as for HCl-doped PANI: the C-C ring stretching vibrations of the

benzenoid ring at 1622 cm-1, the C-H in-plane bending vibrations of the benzenoid rings

at 1191 cm-1, the >C-N+· stretching vibrations of the semiquinone radicals at 1258 cm-1

and the conduction band around 1340 cm-1. Also, it should be noticed that for H2SO4-

doped PANI two closely located Raman bands at 1191 and 1165 cm-1 are observed (Fig.

5.14, curve 3). This is in accordance with the co-existence of the benzenoid and quinoid

units in the oxidized sample.

158


So, taking into account all these results, we can conclude that Raman spectra

completely depend on the oxidation states of PANI, and to a less extent depend on the

acid-dopant.

5.5.2. Raman spectrometry study of the surface composite films

inary, we investigated the Raman behaviour of the pure PA films (Fig.

5.15) in order to compare the vibrational bands of PA in the composite films with those

observed in pure PA. The assignment of the Raman bands for the PM was made

according to the literature [235, 236]. For all types of PA (PA-6, PA-11, PA-12) the

Raman spectra are rather similar and all vibrational bands corresponding to PA are

present. In particular, we clearly observed the C-N-C stretching de at 930 cm-1, the

C-C stretching modes at 950-1150 cm-1, the stretching band of carbonyl group vibration

at 1642 cm-1, the stretching vibrations of the CH2-groups at 2852, 2889 and 2932 cm-1,

and the in-phase twist of the CH2-groups at 1298 cm-1. The spectra also contain the

vibrations of the N-CH2-groups at 2722 cm-1 and the N-H stretching at 3300 cm-1. It

should be noted that in the case of the PA-6 film the vibrational bands, which

correspond to the carbonyl and NH groups are more intense than those observed for the

PA-1 and

3300 cm-1). This observ he molecular structure

of the studied PM (Fig. 2.1), s is changed in the next

row: PA-6<PA-11<PA-12. In bonyl group to CH2-group

for PA-12 is less than that for

Thanks to the confocal micro-Raman Resonance spectrometry, we have shown

n

Prelim

mo

1 and PA-12 films (compare, for example, the peaks’ intensity at 930, 1642

ation is consistent with the difference in t

since the quantity of the CH2-group

other words, the ratio of the car

the PA-6 film.

(chapter 4) that it was possible to mainly observe the bands of PANI in the Raman

spectra of the composite PA/PANI films. Fig. 5.16 shows Raman spectra of the

composite PA-12/PANI films in ES and EB oxidation states. To help making a

comparison, the figure includes also the spectrum of the virgin PA-12 film (Fig. 5.16a,

curve 1) measured under the same conditions. The intensity of the PA-12 spectrum was

multiplied by a factor 10 to observe the position of the vibrational bands.

Fig. 5.16 shows clearly that the Raman spectra of the composite film are

mai ly characterized by the PANI bands. However, it can be noticed that the PANI

bands in this case are shifted in comparison with those of pure PANI (Fig. 5.14).

Specifically, in the spectrum of the doped sample (Fig.5.16a, curve 2) the semiquinone

radical >C-N+· band at 1320-1340 cm-1 is significantly shifted towards 1347-1378 cm-1,

159


Figure 5.15. Raman spectra of the PA films

in the EB oxidation state the band at

1351 cm

n

m the bands are

broader (Fig. 5.16a) than for the PA-6 film (Fig. 5.16b). The other interesting

(λ = 514.5 nm, t = 240 s, p = 1.67 mW):

1 – PA-6; 2 – PA-11; 3 – PA-12

while the dedoped sample (Fig. 5.16a, curve 3) shows a negligible shift of the band at

1165 cm-1 (C-H bending of the quinoid ring) to 1169 cm-1 and the band at 1409 cm-1 (C-

C stretching of the quinoid ring) to 1413 cm-1. Moreover, it should be noticed that on

the Raman spectrum of the composite PA-12/PANI-1 can be observed. This may be explained by the spatial difficulties of the

doping-dedoping processes, because in the case of the composite material the PM

hinders the free diffusion of the ions of the doping or dedoping agents. And this feature

can explain the uncompleted dedoping process of PANI in the composite film. All this

results in the appearance of the >C-N+· stretching vibrations on the Raman spectrum.

In the case of the PA-6 film as a PM (Fig. 5.16b) one can observe similar

behaviour as in the case of the composite film based on PA-12. Namely, on the Rama

spectra of the composite film only characteristic bands of PANI are present (Fig. 5.16b,

curves 2 and 3). For the PA-6/PANI composite film, like for the PA-12/PANI one, the

shift of the vibrational bands is observed. However, for the PA-12 fil

Wavenumber, cm-1500 1000 1500 2000 2500 3000 3500 4000

Ram

an in

ters

ity, a

u..

80000

288928

52

0

20000

40000

60000

3300

2932

2722

164214

427212

98

1064

1114

13

1127

1079

930

3

2

1

160


Figure

Wavenumber, cm-11000 1200 1400 1600 1800 2000

Ram

an in

tens

ity, a

.

80000

1169

1233

161413 14

7315

00

1608

32

1351 70000

u.

5.16. Raman spectra of the composite films based on (a) PA-12 and (b) PA-6

films (λ = 514.5 nm, t = 300 s, p = 0.04 mW):

1 – the pure PM;

2 – the doped composite PM/PANI-HCl film;

3 – the dedoped composite PM/PANI film

30000

40000

50000

6000010

6411

1411

32 1298

1442

1642

1188

1248

1347

1378 15

69

x 1016

05

1152

Wavenumber, cm-11000 1200 1400 1600 1800 2000

Ram

an in

tens

ity, a

.u.

0

1000

2000

3000

4000

5000

6000

7000

× 10

1642

1442

1372

1127

1079

12 13

1

2

1625

1193

55 43

1161

1223

1449

1615

1541

1349

1592

3

3

2

1

(b)

(a)

161


observation is the very strong bands at 1193 and 1625 cm-1 for the doped composite PA-

6/PANI-HCl film (Fig. 5.16b, curve 2). These bands correspond to the C-H in-plane

bending vibrations of the benzenoid rings and C-C ring stretching vibration of the

benzenoid ring, respectively. For the dedoped samples (Fig. 5.16, curve 3) the band at

1161 cm , corresponding to the C-H bending of the quinoid ring is also more intense in

the case of the composite PA-6/PANI film. Such behaviour may be explained by, at

least, two reasons: by a higher quantity of the formed PANI in the PA-6 film and by a

higher doping level of PANI in the case of the PM based on PA-6. Such explanation is

well confirmed by the different thickness of the conducting PANI containing layer (Fig.

4.10). So, these results also indicate the influence of the PM molecular structure on the

properties of the obtained surface composite materials.

5.5.3. Raman spectrometry study of the bulk composite powders

-1

he Raman spectrum of the PA-12 powder is identical to the Raman spectrum

of PA-12 in the film form (Fig. 5.15, curve 3). The Raman spectra of the composite

materials with the d . 5.17. In order to

compare the Raman sp iphatic CH2-groups at

2852, 2889 and 2932 cm ity of the PA powder was

the same for all composite materials.

I)

the char

T

ifferent PANI content are presented in Fig

ectra, the stretching vibrations of the al-1 were normalized, since the quant

Fig. 5.17 shows that the intensity of the vibrational bands in the wavenumber

range 1000-1700 cm-1 increases with the PANI content. But it should be pointed out that

unlike the surface films, in the case of the PA/PANI powders the Raman bands of PA

are also seen in all spectra. Indeed, on the Raman spectrum of the composite powder

with only 0.48 wt.% of the PANI-TSA the bands at 1298 and 1445 cm-1 which

correspond to the CH2-groups vibrations of the PM are observed as well as the C-C

skeletal vibrations at 1067 and 1111 cm-1 (Fig. 5.17a, curve 1).

At the same time, even with such a small PANI quantity in the composite

powder (only 0.48 wt.% of the PANI-TSA complex, i.e. 0.25 wt.% of dedoped PAN

acteristic vibrations of PANI are already observed: at 1195 cm-1 - the C-H in-

plane bending of the benzenoid rings, the >C-N+· stretching vibration of the delocalized

polaron charge carriers – at around 1360 cm-1 and at 1638 cm-1 - the C-C stretching

vibrations of the benzenoid rings. It is established that as the PANI-TSA content

increases the intensity of the PANI bands increases too (Fig. 5.17a, curves 2-6).

162


Figure 5.17. Ram

12/PANI-DBSA powde nt PANI-acid complex content:

a – 1 - 0.49 wt.%; 2 - 1.59 wt.%; 6 - 9.92 wt.%

b - 1 – 0.61 wt.%; 2 – 1.13 wt.%; 5 – 7.75 wt.%

an spectra of the composite (a) PA-12/PANI-TSA and (b) PA-

rs with differe

3 - 3.19 wt.%; 4 - 5.04 wt.%; 5 - 7.53 wt.%;

3 – 3.61 wt.%; 4 – 5.64 wt.%;

Wavenumber, cm-11000 1500 2000 2500 3000 3500

Ram

an in

tens

ity, a

.u.

15000

20000

25000

30000

35000

40000

3300

2889

2852

2932

2727

1625

1193

1344

1064

1295

1442

1636

1440

1195

(b)

5

4

32

1

Wavenumber, cm-11000 1500 2000 2500 3000 3500

Ram

an in

tens

ity, a

.u.

0

10000

20000

30000

40000

50000

1067 1111

1195 1298

1638

2852 28

8929

32

1188 13

4512

54

16221595(a)

1

2

34

56

163


In contrast to the composites doped by TSA, for the composite materials

synthesized in the presence of the other doping acid – DBSA – the PANI characteristic

bands become apparent at a higher PANI-DBSA content (Fig. 5.17b). In order to make

a comparison between these two acid-dopants, the plot of the evolution of the integrated

Raman intensity as a function of the PANI content was made (Fig. 5.18). As one can

see, the evolution isn’t the same – at the same PANI-acid content in the case of TSA the

integrated Raman intensity is higher. Boyer et al [230] studied the evolution of the

Raman spectra of ES with the time of the exposure in HCl vapour or, in other words,

with the increasing of the doping level. They observed a decrease of the C=N bond and

the C-N stretching bands and the appearance of a band assigned to the C-N+· stretching

of the cation radical species at around 1332 cm-1 with the doping level increasing. On

the basis of these results, we can conclude that PANI obtained in the presence of DBSA

has a

ontent, namely, on the interaction of the cation

radical f

s of the aliphatic CH2-

groups.

re after

lower doping level than the one obtained with TSA.

But one should also noticed that the positions of the Raman bands slightly

shifts to lower wavenumbers with the increasing PANI content in the case of both acids.

The more is the PANI content, the lower are the wavenumbers of the characteristic

vibrational bands. The bands shift confirms the above stated interaction of PANI with

the matrix. On the other hand, such evolution shows that the degree of this interaction

strongly depends on the PANI-acid c

ragments of PANI with the electrodonor groups of the PA matrix (Fig. 5.18).

In order to study the influence of additional doping on the composite Raman

properties we collected Raman spectrum of the PA-12/PANI-TSA powder after an

additional treatment with TSA (see Chapter 2) (Fig. 5.19). The spectrum was recorded

under the same conditions (λ = 514.5 nm, t = 300 s, p = 0.167 mW) as the ones used

before for the initial powder, so we can compare the Raman intensity of both spectra.

The spectra were also normalized with the stretching vibration

The figure clearly shows that the additional doping leads to significant

increasing of the all characteristic vibrational bands of doped PANI, namely, the C-H

bending vibrations of the benzenoid ring at 1195 cm-1 and the band at 1345 cm-1 which

is responsible for the >C-N+· cation radical fragments. Such changes in the system may

be easily explained by the increasing of the total quantity of the acid (TSA) and, thus,

they may testify to the presence of the non doped imine sites in the PANI structu

164


Figure 5.18. Evolution of the integrated Raman intensity in the wavenumber range

1047-1680 cm-1 as a function of the PANI-acid content in the composite:

1 – TSA; 2 - DBSA

Figure 5 ectra of

PANI-acid content, wt.% 0 2 4 6 8 10

u.ity

, a.

nten

sg

man

i R

ara

ted

Inte

.19. The influence of the additional doping treatment on the Raman sp

0.0

2.0e+5

4.0e+5

6.0e+5

8.0e+5

1.0e+6

1.2e+6

1.4e+6

1.6e+6

1

2

1

2

Wavenumber, cm -1

the composite PA-12/PANI-TSA (9.92 wt.%):

1 – without additional doping; 2 – with additional doping

1000 1500 2000 2500 3000 3500

Ram

an in

tens

ity, a

.u.

-10000

0

10000

20000

30000

40000

50000

162415

99

1345

1195

1258

1

2

165


the initia

ch

composi

ed that the presence of the PM hinders the formation of PANI with long chains,

which m

l polymerization process. This is also the evidence that the doping agent (TSA)

can be partially removed during washing just after the polymerization process. Such a

result proves unambiguously that an additional doping treatment could be useful to

obtain a higher conducting composite.

On a whole, obtained results show that the composite PA-12/PANI powders

doped by different acids have similar Raman spectra. On these spectra the characteristic

vibrational bands of both PANI and PA are present. However, it can be notices that the

PANI characteristic bands in the composite materials are shifted to the low wavenumber

region in comparison with those of pure PANI. Besides, with increasing the PANI

content in the composites the intensity of the PANI bands increases simultaneously.

5.6. Conclusions

The study of the spectroelectrochemical, thermal, mechanical and structural

properties of the conducting composite materials based on PANI and PA allows us to

come to the following conclusions:

1. It was found that in spite of the preparation method PANI synthesized on the

surface of the bare electrodes or on the surface of the electrodes, covered with the PM

preserves high ability to the reversible redox processes and changes its colour

depending on the oxidation state. This fact gives us the possibility to use su

te films in sensors for different gases, as pH sensors, as electrodes in batteries,

etc.

2. The observed spectroelectrochemical differences between the composite

PM/PANI materials and pure PANI are connected with the diffusion limitations, as in

the case of the PM the counter ions should diffuse from solution to the electrode surface

not only through the formed PANI layer, but also through the polymer film. It was

establish

ay be one of the reasons of the decrease of the PANI conductivity in the

composite materials as compared with that of pure PANI.

3. It was shown that the dopant nature affects the thermal properties of pure

PANI. The thermal stability of both PANI and PA-12/PANI composites is enhanced.

Such composites showed an increase of about 100 0C of the degradation temperature in

comparison with pure PANI independently of the acid-dopant used.

166


4. It was found that the PANI introducing into the PA matrix leads to a

decrease of the tensile strength of the PA films. Also, it was confirmed that the nature of

e acid-dopant is one of the main factors, which influence the mechanical properties of

e final composite film – the tensile strength is higher when using the organic acid-

opants than when using the inorganic ones. Such behaviour is connected with the fact

at organic acids perform triple role – of a dopant, a plasticizer and compatibilizer.

prove mechanical properties of the composite films

ultaneously with their conductivity.

5. It was found that at low PANI-acid content (up to 4 wt.%) the tensile

rength values of the composite films are not far from the values of pure PA. A further

crease of the PANI content caused a decrease of the mechanical properties of the

omposite films independently of the acid used. The main reason is, probably, poor

echanical properties of PANI.

e

pressing posite

PA/PANI material. This fact is ex bility of the acidic hydrolysis of

the PA matrix since during the pressing at higher temperatures the dissociation of the

conducting complex PANI-acid takes place. Thus, in the matrix volume the molecules

of the “free” acid appear which lead to the PA chains hydrolysis and, further, to the

mechanical properties decrease.

. On the basis of the AFM and optical microscopies results, the structural

organisation of the conducting PANI clusters within the insulating PM is found to be

dependent on the PM structure. For example, the final composite material is non-

uniform in the case of the PA-11 and PA-12 films and rather homogeneous in the case

of the PA-6 film.

. The analysis of Resonance Raman spectrometry results has shown that the

composite films of both surface and bulk composites reveal the characteristic

vibrational bands of PANI even at low PANI concentrations. The influence of the PM

structure on the properties of the obtained composite materials was confirmed.

9. It was established that in the course of the formation of the composite

m

confirmed by the Ram

th

th

d

th

Therefore, such dopants im

sim

st

in

c

m

6. It was established that both additional doping and the increase of th

temperature caused the decrease of the mechanical properties of the com

plained by the possi

7

8

aterials the interaction between PANI and the PA matrix takes place. This is

an bands shift to the low wavenumber region.

167

Chapter 6

DIELECTRIC AND

ELECTRICAL PROPERTIES


Introduction

In previous chapters it was stated that the conducting composite PA/PANI

materials have a phase segregation structure, revealed by the presence of clusters of

various conductance values on the composite film surface. Also, it was shown that the

interaction between the PA units and the PANI chains exists. So, we can suppose that

such a cluster organisation of the conducting layer will result in the appearance of

interfacial polarisation effects. Besides, the observed modifications occurring in the

composite materials due to the acid doping also may result in significant changes in the

dielectric behaviour of the doped films. On the other hand, it is well known that the

electrical properties of the composite material are determined by its components, their

conductance and the percolation behaviour.

To confirm the assumption of the influence of the PANI conductivity and its

organisation in the PM on the general conductivity of the composite materials, the

dielectric and electrical properties of the polymer composite materials has been studied

in this chapter.

6.1. Chemically synthesized polyaniline

In order to interpret the obtained data for the composite materials, we have

also studied the dielectric and electrical behaviour of the pure PANI samples in doped

and dedoped forms.

6.1.1. Dedoped polyaniline

Relaxation properties. The frequency dependence of the measured real part ε’

of the complex dielectric permittivity and the real part σ’ of the conductivity for the

dedoped PANI powder (pressed pellet) is presented in Fig. 6.1 at several temperatures.

The spectra in Fig. 6.1 are presented at temperatures higher than 273 K, though the

measurements were performed beginning with 173 K. This is because below 273 K the

value of ε’ is almost temperature-independent having a weak frequency dispersion.

The real part of the permittivity exhibits a relaxation behaviour which

relaxation frequency is temperature-dependent. As one can see, ε’ exhibits a low-

168


Figure 6.1. Frequency dependence of the real part of dielectric permittivity (a) and

conductivity (b) for dedoped PANI at different temperatures. Solid lines are the best fit

to HN equation (2.7)

Frequency, Hz10-1 100 101 102 103 104 105 106 107

σ', S

/cm

10-11

10-10

10-9

10-8

10-7

10-6

273 K 293 K 313 K 333 K 353 K 373 K HN fit

Frequency, Hz10-1 100 101 102 103 104 105 106

ε'

100

101

102

103

273 K 293 K 313 K 333 K 353 K373 K HN fit

(b)

(a)

169


frequency plateau corresponding to the static dielectric constant εs and then decreases

with increasing frequency, forming a low plateau value ε∞ in the high-frequency region

(Fig. 6.1a).

The analysis of the dielectric permittivity using HN equation (equation 2.7)

revealed a temperature dependence of the relaxation time τ (Fig. 6.2), which is well

described by a simple Arrhenius law (see equation 2.8). For the emeraldine base τ0

appeared to be 5.19⋅10-11 s and the activation energy appeared to be 0.539 eV. These

results are in agreement with the calculations performed by Zuo et al [168] for the PANI

powders with different protonation levels. For example, for the emeraldine base they

found 1.6⋅10-10 s and 0.46 eV for τ0 and the activation energy, respectively [168]. At the

same time, the temperature dependent conductivity data obtained (Fig. 6.2) can be fitted

to Arrhenius equation of conductivity:

)exp(0 TkE

B

a−=σσ , (6.1)

where σ0 is the conductivity at very high temperature; Еa is the activation energy; kB is

the Boltzmann’s constant (8.616⋅10-5 eV/K). The activation energy for the conductivity

Figure 6.2. Arrhenius plot of the characteristic relaxation time and of the conducti

was found to be 0.589 eV, which is close to that of the relaxation time (0.539 eV).

vity

for dedoped PANI. Solid line is the best fit to equations (2.8) and (6.1), respectively

1000/T, K-12.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

rela

xatio

n tim

e τ,

s

10-4

10-3

10-2

10-1

100

σ , S

/cm

10-11

10-10

10-9

10-8

10-7

170


As the value of the activation energy is rather high, one can suppose that this

ation process is a β-relaxation, as for the α-relaxation process the activation energy

alue should be much higher and its frequency relaxation ay appear at much higher

mperatures [237]. In other words, the observed relaxation could be a β-relaxation

rocess commonly observed in the traditional polymers since the activation energy is of

e same order. On the other hand, inasmuch as the value of the activation energy for

e relaxation time is close to that of the conductivity (0.539 eV and 0.589 eV,

spectively), we can surmise that the correlation between these processes exists and

at both processes originated from the same transport mechanism. Therefore, the

laxation we observed for dedoped PANI at low frequencies can be the well known

onductivity relaxation [238]. In order to support this assumption the dependence of the

d

(F d

to be nearly equal to -1 with a linearity factor of 0.9951. Besides, it was established that

the relaxation time τ and the dc-conductivity σ obey the following equation [238]:

relax

v

te

p

th

th

re

th

re

c

c-conductivity value is plotted versus the relaxation time in double logarithmic scale

ig. 6.3). The linear behaviour of the dependence was obtained, which slope was foun

)()( 0

TT s

τεε

σ⋅

= , (6.2)

Figure 6.3. Dependence of the characteristic relaxation time versus the conductivity for

dedoped PANI

relaxation time τ, s10-4 10-3 10-2 10-1 100

σ, S

/cm

10-11

10-10

10-9

10-8

10-7

171


where

dc-

conductivity value although not very high

represented in Fig. 6.1a arises from hopping and/or oscillations of these charges around

xed pinn

ε0 is the permittivity of the free space (8.85⋅10-12 F/m). This relation is

characteristic to the conductivity relaxation process

It was observed some deviation between the experimental values and

calculated according to equation (6.2) which can be explained by experimental error –

for example, by the presence of the residual solvent (water) in the sample. Thus, we

measured total conductivity of the sample, not only the conductivity value of PANI.

It should be noted that in conducting polymers is a strong charge trapping [168,

239] and its localized motion when applying an alternating electric field operates as an

electric dipole. The dielectric relaxation in the presence of such an alternating electric

field is a result of charge hopping among available localized sites [174]. So, for PANI

the polarons and bipolarons are the relevant charge species involved in the relaxation

and conductivity mechanisms [168, 240, 241]. At low frequencies, such charge hopping

could extend throughout PANI in the absence of strong pinning, which can lead to a

continuous current, resulting in a dc-conductivity. It should be pointed out that the

charge carriers exist even in the dedoped form of PANI as the sample reveals a

(Fig. 6.1b). The relaxation process

fi ing centres [168].

Thermal behaviour of the conductivity. There have been some reports on the

mperature dependence of the conductivity of PANI. Zuo et al [168] studied the

mperature dependence of the conductivity in the range between 78 and 375 K for

edoped and salt forms of PANI and reported the activation energy of 0.5 eV for the

edoped form. Pingsheng et al [242] showed that the activation energy increased from

.037 to 0.493 eV when the doping solutions pH was varied between 0 and 6.3. Also,

robst and Holze [243] suggested that the method of the aniline polymerization would

e an important factor in explaining the conduction mechanism.

Fig. 6.4 shows the temperature dependence of the conductivity for dedoped

ANI at 100 Hz. As one can see, the first run from 158 K to 433 K is slightly different

rom the others but further temperature cycling of dedoped PANI does not lead to any

urther visible changes in the conductivity value. This observation may be explained by

removed from the pellet. As it was 5.4, PANI in the dedoped state is

te

te

d

d

0

P

b

P

f

f

the fact that during the first increase of temperature the residual moisture (water) is

shown in Fig.

172


Figure 6.4. Temperature cycling of the PANI pellet in the dedoped state at 100 Hz

stable up to 618 K, no further change of the conductivity is observed during next

cycling till 433 K.

Temperature, K

100 150 200 250 300 350 400 450

σ, S

/cm

10-13

10-12

10-11

10-10

10-6

10-9

10-8

10-7

These results confirm the above stated explanation (see section 5.2) that during

the temperature increasing for the dedoped PANI sample only two weight loss stages

are observed – the first one is connected with the moisture evaporation and the second

one is attributed to the decomposition of the PANI backbones (see Fig. 5.4).

6.1.2. Doped polyaniline

Relaxation process. The measured real ε’ and imaginary ε” parts of the

complex permittivity as a function of frequency for the doped PANI samples (pressed

pellets) at 300 K are shown in Fig. 6.5. The doping process of PANI leads to a drastic

increase of the permittivity. We found that the dielectric constant ε’ at low frequencies

is very high, but it drops very fast with increasing frequency for all used acids (Fig.

6.5a). This may be due to the easy charge transfer through well ordered polymer chains

in disordered regions as suggested by Joo et al [244]. This is characteristic of a

conducting polymer and consistent with the literature results [168, 170, 245]. Such

start

173


Figure 6.5. Frequency dependence of the real (a) and imaginary (b) parts of dielectric

10-1 100 101 102 103 104 105 106

permittivity for PANI doped by different acids at 300 K

103

4

5

6

7

8

9

10

11

12

13

1410

10

10

10

10

10

10

10

10

10

10

HClH2SO4

TSA DBSA CSA

Frequency, Hz

ε"

(b)

Frequency, Hz10-1 100 101 102 103 104 105 106

ε'

102

103

104

105

106

107

108

109

1010

1011

1012

HClH2SO4

TSA DBSACSA

(a)

174


behaviou

ies, with a

slope equal to -1 (Fig. 6.5b). Such a dependence indicates that ε” is proportional to 1/f,

and, correspondingly, that the conductivity is constant at low frequencies. This testifies

to the presence of a dc-conductivity. Si

looks like a relaxation process (Fig. 6.5a). Such behaviour is characteristic of the charge

carriers systems [174]. The localized charge carriers when applying an alternating

electric field can hop and jump to neighboring localized sites which form a continuous

connected network, allowing the charges to travel through the sample and, thus, causing

the electric conduction.

In order to study in detail the relaxation and conduction mechanisms in the

PANI samples we also performed measurements in the high frequency region (from 106

Hz till 109 Hz) (Fig. 6.6). As one can notice from Fig. 6.5a and 6.6, the relaxation

process, which for dedoped PANI was observed in the low frequency region (Fig. 6.1),

shifts toward higher frequencies as a result of the doping process. Moreover, it is

established that the position of the relaxation frequency of doped PANI is dependent on

the used acid-dopant.

Therefore, we can say that a relaxation process is present in the studied doped

PANI samples and it is the interfacial polarization process, which for HCl and TSA

doped PANI is observed in the low- and in the high-frequency regions, respectively.

This supposition is in good agreement with the obtained values of conductivity – PANI

doped by TSA has a higher conductivity value than sample doped by HCl (0.031 S/cm

and 0.49 S/cm for HCl- and TSA-doped PANI, respectively). According to the literature

[171], the increase in the counter-anion size leads to an increase of the interchain

r can be also explained as one due to the increased contribution of the dc-

conductivity [174].

Furthermore, the dependence of ε” versus frequency in the double logarithmic

scale for all PANI doped samples is almost a straight line at low frequenc

milar behaviour was also observed by other

researchers, - for example, Lian et al for poly(N-alkylanilines) [170] and Chen et al for

poly(3-alkylthiophene)s [245]. So, according to Fig. 6.5b, the dc-conductivity value of

HCl doped PANI is lower than that of TSA doped PANI.

As it was mentioned above, ε’ increases with the frequency decrease for all

acid-dopants and the very high polarization may hide the relaxation process if any in the

studied low frequency range at room temperature. Nevertheless, we can say that the

feature of ε’ decreases in the high-frequency region for HCl and CSA doped PANI

175


dielectric permi )

Figure 6.6. Frequency dependence of the real (a) and imaginary (b) parts of the

ttivity for PANI-TSA. Solid lines are the best fit to HN equation (2.7

(a)

Frequency, Hz106 107 108

ε'

60

80

100

120

140

160

180

200

220

240173 K193 K213 K233 K253 K273 K293 K313 KHN fit

(a)


ε"

100

101

102

103

104

173 K 193 K 213 K 233 K253 K273 K293 K313 KHN fit

(a)

(b)

176


distance. This makes the hopping between chains more difficult and, hence, results in a

reduction of conductivity. But, on the other hand, the doping by TSA causes a compact

coil conf

6.7, respectively. As one can

see, the

e of the process and the conductivity values was found (Fig.

6.8).

ormation of the PANI chains [246]. Such conformation leads to the possibility

of the electron hopping not only along the PANI chains but also between the chains,

which may significantly increase the conductivity.

In order to study the relaxation behaviour with the temperature change and to

estimate the activation energy of the process, dielectric measurements were performed

for TSA-doped PANI in the high frequency range. The real and imaginary parts of

dielectric permittivity and real part of the conductivity for the PANI-TSA sample as a

function of the temperature are depicted in Fig. 6.6 and

real part of the dielectric permittivity exhibits a relaxation process of a rather

high dielectric strength (Fig. 6.6a), as a result of the doping process. This relaxation

process is associated with the charge carriers’ mobility. However, for dedoped PANI

form only a flat dielectric response was observed in this frequency region.

As in the case of dedoped PANI, the temperature dependencies of the

relaxation time and dc-conductivity for doped PANI may be described according to the

equations (2.8) and (6.1). It was found that the activation energy for PANI-TSA is 0.093

eV, i.e. much lower than that for the dedoped sample (0.539 eV). Also, the correlation

between the relaxation tim

Conductivity properties. As expected, the doping process is seen to lead to an

increase of the conductivity at low frequencies (Fig. 6.9). Analyzing the obtained results

we can say that at low frequencies the real part of the conductivity spectra of doped

PANI are dominated by a dc-conductivity, whereas at high frequencies the spectra

reveal an ac-conductivity increasing with frequency (Fig. 6.7).

One should notice the observed difference between conductivity values

measured at low and high frequencies. Such behaviour may be explained by the use of

two different measurement methods – low- and high-frequency devices (see section

2.3.5).

We have undertaken the measurements of the temperature dependence of the

electrical conductivity for the PANI samples doped by the different acids. Fig. 6.10

shows the obtained conductivity variation as a function of the temperature. Zuppiroli et

al [247] assumed that dopant counter ions play a very active role in the formation of the

177


Figure 6.7. Real part of the conductivity for PANI-TSA as a function of temperature.

TSA doped PANI

Solid lines are the best fit to HN equation (2.7)

Figure 6.8. Dependence of the characteristic relaxation time versus the conductivity for


σ', S

/cm

10-5

10-4

10-3

10-2

173 K193 K 213 K 233 K253 K 273 K 293 K313 K HN fit

relaxation time τ, s10-10 10-9 10-8 10-7

σ', S

/cm

10-5

10-4

10-3

10-2

178


Figure 6.9. Frequency dependence of the real part of the conductivity for PANI doped

by different acids at 300 K

conducting clusters by acting as bridges between neighbouring chains and stabilizing

the conducting regions. As can be seen from this figure, the conductivity initially

increases gradually with the rise

Frequency, Hz10-1 100 101 102 103 104 105 106

σ', S

/cm

10-10

10-9

10-8

10-710-2

10-1

100

in temperature from 173 K to ∼320 K and then

decrease

ility of organic acids in

compari

ion. The first process is likely to be responsible for the conductivity changes in

the vicin

s. It should be noted that for the PANI sample doped by organic acids (TSA,

DBSA and CSA) the maximum value is reached at ∼340 K for TSA and DBSA and at

∼380 K – for CSA-doped PANI. For the PANI samples doped by inorganic acids (HCl

and H2SO4) the decrease of the conductivity starts earlier (300 and 330 K for HCl and

H2S04, respectively). This is connected with higher thermal stab

son with the inorganic ones (see section 5.2).

There are three potential causes for the change in the conductivity: (1) the loss

of the adsorbed moisture; (2) the gradual deprotonation and (3) the oxidative

degradat

ity of 350 K. Other two processes lead to the conversion of conducting PANI

into non-conducting one and a consequent loss of conductivity is anticipated and indeed

observed. This decrease in conductivity is attributed to the separation of the dopants

from the polymer backbones, which results in the dedoping process and, thus, a large

PANI-HClPANI-H2SO4

PANI-TSAPANI-DBSA PANI-CSA dedoped PANI

179


Figure 6.10. Tempe

from e

m tivity studies

the m

sam

ents of

the conductivity dependence on the frequency

sample was being held at a certain tem me time (∼20 min) until

measurements at all frequencies were done. In the second case (Fig. 6.11) we measured

rature dependence of the electrical dc-conductivity for PANI doped

with different acids

portion of imine nitrogen is being in the non-conducting state. In its turn, this leads to a

decrease in the concentration of polarons. A support for the above inference is obtained

the thermogravimetry results (see section 5.2) which indicate the presence of th

different weight loss stages for the doped PANI samples. The assumption about the

decreasing of the polarons concentration is also well confirmed by the EPR

easurements performed by Kuo and Chen [248]. They performed conduc

for PANI doped with diphenyl phosphate and found that the temperature dependence of

the spin density is consistent with that of the conductivity [248].

In order to verify the proposed assumption about the deprotonation and loss of

oisture we performed additional conductivity measurements for the PANI-HCl

ple as a function of temperature at a fixed frequency (100 Hz) (Fig. 6.11). An

observed difference between conductivity values in Fig. 6.10 and 6.11 is connected with

the use of other way of measurement. In the first case we performed measurem

and temperature simultaneously, i.e. the

perature for so

Temperature, K150 200 250 300 350 400 450

σ', S

/cm

10-4

10-3

10-2

10-1

100

101

HClH2SO4 TSA DBSA CSA

180


eans that not only the moisture

was eva

Figure 6.11. Temperature dependence of the electrical conductivity for the PANI-HCl

pellet at 100 Hz

the conductivity at a fixed frequency with temperature varying continuously with a

temperature ramp of 2 0C/min. Therefore, the conductivity values obtained by the latter

method are higher than those obtained by the former method. But, in spite of the slight

difference in conductivity, the common behaviour is the same in both cases.

So, as one can see, the first scan is rather different from the next ones (Fig.

6.11). The conductivity maximum, which is observed at ∼310 K is connected with the

beginning of the moisture evaporation and is in agreement with the thermogravimetric

measurements (Fig. 5.4). The conductivity of PANI registered during the next scans is

significantly less compared with the first one, which m

porated, but also the doping acid – HCl, which is a rather volatile acid indeed.

But, nevertheless, some acid remains in the sample, since the conductivity value is still

rather high in comparison with dedoped PANI (Fig. 6.4).

The obtained conductivity measurements are also in agreement with the

investigation of Prokes et al [249], who studied the resistivity evolution of the PANI

films and powders. They found that the resistivity value increased when exposed to high

temperatures. Also, they performed resistivity measurements during the temperature

Temperature, K100 150 200 250 300 350 400 450

σ, S

/cm

10-5

1 -4

10-3

10-2

1 -1

1 00

0

0

start

181


cycling [249]. The initial increase of the resistivity in the first run reflects the loss

water, while the other ones are connected with the deprotonation and a potential

degradation.

In order to calculate the activation energy value, the dependence of log

versus T-1 was plotted for the linear part of the temperature dependence of the

conductivity for doped PANI (Fig. 6.12). This plot indicates an increase in conductivity

with temperature that may be due to a hopping mechanism [127]. The linear variation of

the conductivity versus temperature for all acid-dopants suggests a thermally activated

process of an Arrhenius-type in the experimental temperature range. The values of the

activation energy calculated from the slope (equation 6.1) are listed in Table 6.1. The

obtained value for the PANI-HCl pellet is in good agreement with the activation energy

value obtained for the PANI derivative – poly(N-methylaniline) doped with chloride io

(0.11 eV) [250].

of

σ

n

significantly lower than that for dedoped PANI. Such difference can be explained by the

Figure 6.12. Arrhenius plot of the conductivity for the doped PANI samples

The obtained values of the activation energy for the doped PANI samples are

presence of localized electrons in dedoped PANI which after the doping process are

delocalized and, therefore, the conductivity process is facilitated.

1/T, K-10.0035 0.0040 0.0045 0.0050 0.0055 0.0060

1

100

σ, S

/cm

0-4

10-3

10-2

10-1

101

HClH2SO4

TSADBSACSA

182


Table 6.1. Activation energy for PANI doped with different acids

Acid-dopant Activation energy Ea, eV

HCl 0.076

H2SO4 0.159

TSA 0.080

DBSA 0.187

CSA 0.118

dedoped 0.539

The obtained information of relaxation process and conductivity behaviour for

pure PANI is of great importance for a better understanding of the phenomena observed

in PA/PANI composite materials discussed in the next sections.

6.2. Surface conducting composite materials

As it was shown in section conductive composite films have a

layered film structure and a PANI clusters organisation. This can result in the

5.4, the surface

appearance of interfacial polarization effects resulting in the appearance of the

relaxation processes. Additional relaxation associated with the dipolar reorientation of

chains segments or side groups in the PM may contribute the whole spectra of the

composite film.

In order to distinguish these relaxation processes from those of the PM we have

first performed dielectric measurements for the pure PA films.

6.2.1. Dielectric properties of the polyamide matrices

As it is known [174, 237], both dielectric constant and loss factor depend upon:

- the dipole density, characterized by the total number of dipoles per volume

unit;

- the ability of the dipoles to follow the reversal of polarity when applying an

alternating electrical field. Moreover, the mobility of the dipole, in turn, depends upon

the mobility of the polymer chains to which the dipoles are attached.

183


At very low temperatures, below the glass transition temperature, the dipolar

activity of the polar materials is reduced. As the temperature increases, the mobility of

the dipole and, consequently, the permittivity increase.

known to be a γ-relaxation. About 193 K a polarization process, characterised by a

lecules

the

diele tion of

β-

com

K,

trans , which

ε”

p increase with the increase in temperature as a consequence of

interfacial polarization contribution and ionic conductance (Fig. 6.13b). Therefore, the

third two

relaxation p β γ

follow Arrhenius law (equation 2.8) and the corresponding activation energies are listed

The dielectric properties of most polyamides have been examined and detailed

information can be found in McCrum et al work [237]. Dielectric spectra obtained for

PA-6, PA-11 and PA-12 in this study are shown in Fig. 6.13, 6.14 and 6.15,

respectively. The solid lines are the best fit to HN equation (2.7). For all types of PA at

least two polarization processes related to molecular motions are generally observed in

the investigated temperature range. At lower temperatures (about 173 K) a polarization

process of weak intensity can be seen at high frequencies (Fig. 6.13-6.15). This

relaxation is thought to be due to the local motion of chain segments, mainly, (CH2)

segments, located between the interchain hydrogen bonds [237, 251]. This relaxation is

undergoes a shar

symmetric distribution of relaxation time (α = 0.5, β = 1), is observed which is related

to a mechanism of motion (rotation) of the amide bonds together with water mo

that are bonded to them – β-relaxation process [237]. It should be also noticed that

ctric γ-relaxation is weak, whereas the β-relaxation, which involves the mo

the polar amide groups, is much more pronounced in the dielectric spectra (Fig. 6.13-

6.15). As one can see, the film of PA-6 (Fig. 6.13) displays an extremely broad

relaxation that may be connected with a higher content of the amide groups in

parison with PA-11 and PA-12 (Fig. 2.1).

It has been also shown in the literature that at higher temperatures (313-343

depending on the type of PA) a strong relaxation is found, which corresponds to the

glass-rubber transition of the amorphous phase (α-relaxation) [237]. Above this

ition temperature the materials show a sharp increase in dielectric constant

is due to an interfacial polarization process as a result of trapping of free charge carriers

at boundaries between crystalline and amorphous regions [174]. In our case

relaxation process (α-relaxation) is overlapped and we can distinguish only

rocesses - and ones.

The temperature dependences of these relaxations (β and γ) were found to

184


Figu

several temperatures

re 6.13. The dielectric loss ε” of the PA-6 film as a function of frequency for

Frequency, Hz10-1 100 101 102 103 104 105 106

ε"173 K

β 193 K213 K

0.08

233 K

0.02

0.04

0.06

0.00γ

6

Frequency, Hz10-1 100 101 102 103 104 105 106

313 K333 K353 K 5373 K

1

2

3

4

ε"

β(b) 0

(a)

185


eral temperatures

Figure 6.14. The dielectric loss ” of the PA-11 film as a function of frequency for

several temperatures

Frequency, Hz10-1 100 101 102 103 104 105 106

ε"

0.00

0.02

0.04

0.06

0.08

0.10173 K193 K213 K 233 K

γ

β

ε

Figure 6.15. The dielectric loss ε” of the PA-12 film as a function of frequency for

sev

Frequency, Hz10-1 100 101 102 103 104 105 106

ε"

0.00

0.02

0.04

0.06

0.08173 K193 K213 K 233 K

β

γ

186


in Tab

atic

copoly

Table 6.2. The activation energies of polyamides

Activation energy Ea, eV

le 6.2. The calculated activation energy for the γ process is in good agreement

with previously reported activation energy for PA-12 [237] and for the aryl-aliph

amides [251].

Type of PA γ β

PA-6 0.326 0.525

PA-11 0.436 0.737

PA-12 0.379 0.688

The general trend in the dielectric data for all studied types of PA is quite

similar, but a few differences are observed. The loss peak of the β process is more

intense in PA-6 than in PA-11 and PA-12. This is in agreement with the higher

concentration of the amide groups in PA-6. A freedom of motion of these amide bonds

was assumed to be the cause of this relaxation process [237].

6.2.2. Dielectric and electrical properties of the surface conducting

composite based on the PA-12 matrix

Relaxation properties. Fig. 6.16 presents the frequency dependences of

” for the PA-12/PANI film in the doped and dedoped states. The spectrum of the virgin

PA-12 film is given for comparison. The solid lines in both ε’ and ε” are f

easured data using HN function (equation (2.7)). It should be noticed that spec

Fig. 6.16 are depicted at 193 K since at higher temperatures the α-relaxation process

ε’ and

ε

its to the

m tra in

and nic conductivity den all relaxations.

The results indicate that the aniline polymerization inside the PA film and

PANI presence in the film causes a great increase of ε’ and ε” in the whole studied

frequency region. This feature is observed for the composite films with PANI in the

dedoped state as well as in the doped one. This may be due to the additional polarization

which emerges from the presence PANI even in the dedoped state.

As it has been seen (see section 6.1.1) the value of ε’ is rather high even in dedoped

PANI. Analyzing the obtained results we can conclude that the presence of PANI (12

wt.%) even in thin subsurface layer leads to considerable changes in the dielectric

io hid

of charge carriers in

187


Frequency, Hz

Fi s

a function of the frequency for ANI film (12 wt.%) at 193 K

gure 6.16. The variation of dielectric constant ε’(a) and dielectric loss factor ε” (b) a

the composite PA-12/P

10-2 10-1 100 101 102 103 104 105 106 107

ε'

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5PA-12 PA-12/PANI dedopedPA-12/PANI doped HCl PA-12/PANI doped HClO4 HN fit

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107

ε"

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

PA-12PA-12/PANI dedopedPA-12/PANI doped HCl PA-12/PANI doped HClO4

HN fit

τ1

γ

β

(b)

(a)

188


spectra.

In the virgin PA film and dedoped composite film only two relaxation

processes are observed. The first one - the high frequency process - is attributed to the γ-

relaxation process associated with the local chain motion. The second process is most

likely correspondin metric distribution

of relaxation time (α ∼0.5, β 1) and by a relat y (Table 6.2).

In the doped PA-12/PANI film the great increase in ε’ (Fig. 6.16a) can be

explained by an additio larization caused high concentration of charge

carriers in conductive PANI particles dispersed within the insulating PM. As the

conductivity of the dedo -12/PANI film is closer to the virgin PA-12 film than

that of the doped PA-12/PANI, such great variation of ε’ and ε” was not observed in the

former film

s to

the

relaxation process labelled efore, the layered structure of the film

and the PANI clusters organisation in the subsurface conducting layer may result in the

appearan

rature (Fig. 6.17) showed that at high

temperat

g to a β-relaxation process characterised by a sym

= ively high activation energ

nal po by a

ped PA

, i.e. in the dedoped PA-12/PANI film. A similar behaviour was observed by

Mattoso et al [252] in the case of the PVDF/poly(o-methoxyaniline) (PVDF/POMA)

blends, which were prepared by casting the blended solution of POMA and PVDF in

dimethylacetamide.

On the contrary, the doping process (irrespective of the doping acids) lead

appearance in the studied frequency and temperature ranges of an additional

τ1. As it was stated b

ce of polarisation phenomena. Hence, this additional relaxation τ1 can be

attributed to the interaction between PA and doped PANI. It was established that the

additional relaxation process is present only when PANI is in its conducting doped

state.

The performed dielectric relaxation measurements for the composite film

doped by HCl as a function of the tempe

ures (more than 263 K) all relaxations are hidden or overlapped by the

conductivity and α-relaxation process associated with the chain motion in the polymer

matrix, which dominate the spectra. As one can see in Fig. 6.17, the relaxation process

τ1 is temperature-dependent. The characteristic relaxation time τ1 was taken at the

position of the maximum of dielectric loss and its temperature dependence was

analyzed according to equation (2.8). The obtained dependencies are shown in Fig. 6.18

and the obtained fit parameters can be found in Table 6.3. As one can see from the

obtained results, the activation energy of the relaxation process τ1 have rather small

189


Figure. 6.17. The frequency dependencies of ε” for the composite PA-12/PANI

(12 wt.%) film doped by HCl.

Figure 6.18. Arrhenius plots for the relaxation processes observed in the doped

composite PA-12/PANI films

1000/T, K-1

3.5 4.0 4.5 5.0 5.5 6.0

rela

xatio

n tim

e τ,

s

10-8

10-7

10-6

10-5

rela

xatio

n tim

e τ,

s

10-4

10-3

10-2

10-1

100

101

τHCl

τHClO4τβ

Frequency, Hz10-1 100 101 102 103 104 105 106

ε"

0.00

0.05

0.10

0.15

0.20

0.25

0.35

0.30

173 K193 K213 K 233 K 253 K HN fit

190


values in comparison with β-relaxation. As it was established previously, the surface

composi

Tabl

te PA-12/PANI film consists of two layers - the first one being the conducting

doped PA/PANI layer and the second one being the pure PM (see section 4.3). This

interfacial polarisation leads to a Debye type relaxation process [174]. Therefore, it may

be suggested that the charge carriers are responsible for this phenomenon.

e 6.3. The fit parameters according to Arrhenius equation (2.8) for the doped

PA-12/PANI films

HCl HClO4Acid-dopant

Parameter τ1 τβ τ1 τβ

τ0, s 2⋅10-9 7.14⋅10-14 1.03⋅10-11 7.14⋅10-14

E, eV 0.115 0.536 0.174 0.536

Conductivity. It should be noticed that not only dielectric, but also electrical

properties of the film are changed after the polymerization and doping processes since

the composite PA-12/PANI film after the doping process gains conductive properties.

Indeed, the surface conductivity measurements performed using four-electrode method

have demonstrated that the dc-conductivity of the PA-12/PANI-HCl film is higher

(namely 5⋅10-5 S/cm at 303 K) than that of pure PA-12 (10-14 S/cm). On the other hand,

it is less than the conductivity of pure PANI (3.3⋅10-2 S/cm), which is explained by the

low PANI content in the composite film (12 wt.%). The performed measurement of the

surface resistance of the composite film doped by HCl is shown in Fig. 6.19, curve 1.

The resistance increases with the decrease of temperature, clearly implying that the

obtained material has to be considered as a semiconductor in the whole range of

temperatures. The activation energy for the conduction as derived from the slope of

Arrhenius plot (Fig. 6.19, curve 2) was estimated to be 0.075 eV which is similar to the

values obtained for the pure PANI powders (Table 6.1). Specifically, for PANI-HCl the

activation energy was found to be 0.076 eV.

Also, we should conclude that the choice of the dopant is very important. The

activation energy is slightly higher in the case of using HClO4 acid as a dopant in

comparison with that for HCl-doped PANI (Table 6.3). Such a feature implies that, in

the case of this acid, the conductivity should be less than in the case of HCl. And, really,

the surface conductivity of the samples doped by HCl is 5⋅10-5 S/cm and for that doped

191


200 220 240 260 280 300

Res

istan

ce R

, Ohm

10

10

106

5

4

Temperature, K

Figure 6.19. Temperature dependence of the resistance (1) and the corresponding

Arrhenius plot (2) for the composite PA-12/PANI-HCl film

by HClO4 is 6⋅10-6 S/cm (measured by four-electrode method).

6.2.3. The influence of the polymer matrix structure

As it was previously shown (chapter 3.5) the structure of the PM can

significantly vary the properties of the final composite film. In order to verify the

influence of the PM on the dielectric and electrical properties we examined the

composite films based on PA-6 and PA-11.

In Fig. 6.20 the frequency dependence of ε” for the composite PA/PANI films

are plotted. The spectrum of the virgin film is given for comparison. The figure clearly

shows that for both types of polyamide (PA-6 and PA-11) the dielectric behaviour of

the dedoped composite films is similar to that of the PA-12/PANI film (Fig. 6.16). The

presence of dedoped PANI in the composite film leads to some increase of the β- and γ-

relaxations strength.

nges significantly the dielectric spectra of the

composites. Irrespective of th

The PANI doping process cha

e PM, the doped composite films preserve the relaxation

1000/T, K-1

3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

1/R

, Ohm

-1

10-6

10-5

10-4

1

2

192


Figure 6.20. The variation of dielectric loss factor ε” as a function of the frequency for

the composite PA-11/PANI (a) and PA-6/PANI (b) films at 193 K

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107

ε"

0.00

0.02

0.04

0.06

0.10

0.12

0.08

PA-6PA-6/PANI dedopedPA-6/PANI doped HClHN fit

β relaxation γ relaxation

τ1

τ2

Frequency, Hz

ε

10-2 10-1 100 101 10 103 104 105 106 1070.00

2

"

0.02

0.04

0.06

0.08PA-11PA-11/PANI dedopedPA-11/PANI doped HCl HN fit

τ1(a)

β relaxation

γ nrelaxatio

(b)

193


process which is caused by β-relaxation of PA. But the doped composite films also

revealed an additional relaxation process τ1 in the investigated temperature and

frequency ranges. The additional relaxation process is, obviously, connected with the

presence of charge carriers in the PANI containing layer.

It should be pointed out that the spectra in Fig. 6.20 are presented at 193 K,

since above the glass transition temperat

respectively) these relaxations are no longer visible because of the appearance of the

ionic conductivity and the α-relaxation process, which dominate the spectra.

Moreover, it should be noted that the composite PA-6/PANI film (Fig. 6.20b)

exhibits somewhat different behaviour. The doping by HCl leads to the appearance of

two additional relaxation processes. These peaks are labelled τ1 and τ2.

The temperature dependencies of these relaxation peaks for both PA are

presented in Fig. 6.21 and the fit parameters obtained by fit to Arrhenius equation (2.8)

for the doped films are shown in Table 6.4.

Table 6.4. The fit parameters for the doped PA/PANI films

ure (323 K and 313 K for PA-6 and PA-11,

PA-6 PA-11 Type of PA

Parameter τ1 τ2 τβ τ1 τβ

τ0, s 1.27⋅10-6 1.2⋅10-8 5.79⋅10-18 4.37⋅10-10 4.48⋅10-15

E, eV 0.039 0.052 0.673 0.149 0.555

As we can see from the obtained results, the second peak (τ2) observed in the

composite film based on PA-6 has an activation energy of 0.052 eV. This value is found

to be closer to the one found in pure PANI and, therefore, the second relaxation peak

may be attributed to the interfacial polarization in PANI, as it was discussed previously

(see sect

may be explained by the higher conductivity values obtained in

the PA f

ion 6.1). On the other hand, analyzing the shape and the obtained values of the

activation energy for τ1 for PA-11 and PA-6 (Table 6.4) one can found that the

relaxation peak τ1 for the former is broader than for the latter.

Also, it should be noted that in the PA based composite films the relaxation

process are found at higher frequencies compared with the composites based on the PET

films [223]. This fact

ilms in comparison with those for the PET film. It is known that the relaxation

frequency is proportional to the conductivity. So, the increase of frequency by four

194


Figure 6.21. Arrhenius plot for the relaxation processes observed in the doped

1000/T, K-14.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

rela

xatio

n tim

e τ,

s

10-7

10-6

10-5

10-4

rela

xatio

n tim

e τ,

s

10-4

10-3

10-2

10-1

100

101

τβ

τ1

(b)

1000/T, K-1

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

rela

xatio

n tim

e τ,

s

10-4

10-3

10-2

10-1

100

101

rela

xatio

n tim

e τ,

s

10-8

10-7

10-6

10-5

10-4

τβ

τ1

τ2

(a)

composite PA-6/PANI (a) and PA-11/PANI (b) films

195


decades signifies the increase of the conductivity value by four orders. Indeed, the

resistance of the subsurface conductive layer of the PA/PANI film is four orders lower

than that in the PET/PANI film case.

According to the variation of the activation energy depending on the t

PA (Tab

ype of

osite films

should v

ore conductive than the one based on PA-

12 – the variation of the activation energy is shown in Table 6.5. This result is contrary

to the data ob ut, as it was

mentio e relaxation peak the case of the PA-11/PANI composite is

ra rding to this observation we may believ that the re ation

process this cas ve two relaxation processes. So, the hidden

relaxatio ess could the o d disa nt in th

Table 6.

le 6.3 and 6.4) the studied composites can be placed in the following row:

PA-6/PANI-HCl > PA-12/PANI-HCl > PA-11/PANI-HCl > PA-12/PANI-HClO4

It is logical to suppose that the conductivity value of these comp

ary in the same row. In order to check this assumption we measured by the

four-electrode method the resistance value of the composite films doped by HCl (Fig.

6.22).

As one can see from this figure, the composite film based on the PA-6 film is

the most conductive one, which is in agreement with the obtained row. But it turned out

that the composite film based on PA-11 is m

tained from the relaxation processes (Table 6.3 and 6.4). B

ned above, th τ1 in

ther broad and acco e lax

τ1 in e is an o rlap of

n proc explain bserve greeme e results.

5. The activation energy for the doped PA/PANI films calculated from Fig. 6.22

Type of PA Activation energy, eV

PA-6 0.051

PA-11 0.064

PA-12 0.079

We can say that in spite of the fact that the surface conductivity value of the

composite surface conductive films can not be measured using DRS, the conductive

properties of these films can be evaluated through the study of the interfacial relaxation

processes in the films.

196


Figure 6.22. Arrhenius plot for the conduction of the composite films, based on the

different types of PA:

1 – PA-6/PANI-HCl film;

2 – PA-11/PANI-HCl film;

3 – PA-12/PANI-HCl film

6.3. Bulk conductive composite materials based on polyaniline and

polyamide

In the previous chapter (section 4.5) it has been shown that it is possible to

prepare conducting composite particles with a PA core and a PANI shell (Fig. 4.18).

Such composite organization may result in the appearance of polarisation phenomena,

as it was shown for the surface conductive composite films (section 6.2).

In this subsection we have focused our attention on the bulk conducting

composite materials based on PA-11 and PA-12 powders as well as on the conducting

films obtained by compression-molding. Special attention is paid to the influence of the

doping acid and to the influence of the PANI-acid amount on the electrical and

dielectric properties of the composites.

1000/T, K-13.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

1/R

, Ohm

-1

10-6

10-5

10-4

1

2

3

197


6.3.1. Effect of the dopant nature on the properties of the composite

PA-12/PANI powders

As it was widely discussed in the literature [253, 254] and in the previous

chapters, the nature of the counter ions is one of the determining factors controlling the

conductive properties of the composite materials such as the level and stability of the

conductivity. But, apart from the direct incorporation during the polymerization process,

the counter anions can be changed by ion-exchange after the formation of PANI. We

used this approach in the preparation of the samples doped by different inorganic and

organic acids. It should be noticed that the PA particles used in this study were

polydisperse (14-243 µm) and the PANI content was 2.38 wt.% counting on dedoped

PANI.

Relaxation properties. Preliminary we performed measurements for the virgin

PA-12 powder and dedoped PA-12/PANI composite. One can notice that as for the

surface conducting composite films, for the bulk composites only an increase of the β-

relaxation strength is observed in the dedoped samples in comparison with virgin PA

powder (Fig. 6.23).

ANI in the composite material leads to a great increase

in ε’ and ε” for the low-frequency range (<106 Hz), which may be associated with a

greater interfacial polarization due to a higher electrical conductivity. The results

obtained for other composite materials, for example for the composite PVDF/POMA

films [252] and for the TiO2/PANI nanocomposites [255], confirm such an explanation.

ecause of the very high values of conductivity, the dielectric permittivity

spectra of ε’ and ε” do not reveal any relaxation peaks at all studied temperatures. The

dipolar relaxation processes of PA are hidden by the large polarization of the doped

composite. Therefore, we also performed measurements in the high frequency region

(from 10 9 Hz). We found that virgin PA-12 did not reveal any relaxation in

this ran the PA-12/PANI-TSA composite a well-defined relaxation

process is observed followed by a dc-conductivity (Fig. 6.24). The peak frequency is

practically stable with increasing the temperature.

t is

shown in Fig. 6.2 f the activation

nergy is determined (0.032 eV). The value of activation energy

The presence of doped P

B

6 Hz till 10

ge, whereas in

Arrhenius plot of the relaxation time τ for the PA-12/PANI-TSA pelle

5. A straight line behaviour is obtained and the value o

e

198


Frequency, Hz10-2 10-1 100 101 102 103 104 105 106

ε"

10-2

10-1

100

101

102

PA-12PA-12/PANI dedoped

Figure 6.23. The variation of dielectric loss factor ε” as a function of the frequency for

virgin PA-12 and for the dedoped PA-12/PANI composite at 303 K

Figure 6.24. Imaginary part of dielectric permittivity as a function of frequency for the

PA-12/PANI-TSA powder

Frequency, Hz106 107 108 109

ε"

0

20

40

60173 K193 K 213 K 233 K 253 K 273 K 293 KHN fit

199


Figure 6.25. Arrhenius plot of dielectric relaxation time for the


for the PA-12/PANI-DBSA samples is 0.065 eV. The higher activation energy for

PANI-DBSA is in agreement with the lower conductivity in this case in comparison to

the conductivity of the PA-12/PANI-TSA composite (see below).

observed at higher fre y be d ers’ mobility. The

presence of the charge carr observed only in the doped PANI complex, as for the

composite material with dedoped PANI the high-frequency region reveals only flat

dielectric response. The ment of this rel process to a rather high

frequency in comparison with surface composite materials (Fig. 6.16 and 6.20) is

connecte

Analyzing all obtained results we can suppose that the relaxation process

quencies ma ue to the charge carri

iers is

displace axation

d with the higher conductivity value obtained for the bulk composite PA-

12/PANI-acid material.

Conductivity. The dc-conductivity value -14

-13

is found to be 1.39⋅10 S/cm and

5.23⋅10 PA-12 and for the dedoped PA-12/PANI sample, respectively. The

slightly higher conductivity value for the dedoped sample in comparison with pure PA-

S/cm for

1000/T, K-13.0 3.5 4.0 4.5 5.0 5.5 6.0

xatio

n τ

, s ti

me

rela

10-9

10-8

200


12 can be explained by the presence of charge carriers. As it was shown with the help of

Raman spectrometry, even in the dedoped basic form of PANI these charge carriers

exist (the presence of the conduction band on the Raman spectrum of dedoped PANI

(Fig. 5.14)).

After the doping process the value of dc-conductivity increased greatly by

more than 10 orders of magnitude (Fig. 6.26) in spite of the fact that only 2.38 wt.% of

dedoped PANI was in the composite. As one can see from this figure, the most

conductive is the sample doped by TSA. In order to calculate the activation energy, the

measurements of conductivity as a function of temperature for all dopant-acids were

performed (Fig. 6.27). It was found that the conductivity of the composites increases

with temperature and then passes two successive maximums at 293 K and 383 K. A

linear behaviour is observed for the increase in conductivity with temperature for T <

293 K. Above this temperature, the conductivity decreases with increasing temperature

up to 343 K, then a new increase of the conductivity to 383 K is observed. A final

ecrease of the conductivity is observed at temperatures above 383 K. The first decrease

in conductivity can be related to the residual moisture loss. As it was shown by

thermogravimetric analysis (Fig. 5.6), the first weight loss started practically at room

temperature, which is synonymous with the loss of moisture hydrogen-bonded with

PANI. As one can see, in the case of DBSA-dopant this decrease is the smallest (Fig.

6.27). According to [169], on account of the hydrophobic nature of the long alkyl chain

of DBSA ess of the moisture to the PANI chain is difficult. The presence of the

moisture leads to the charge delocalization, most probably by salvation of the acid

anions and, thereby, reducing the electrostatic interaction between the positive charge

and the anions [169]. This charge distribution due to the moisture presence has been

indicated to reduce polarization effects of the anion [169]. The decrease in conductivity

value is assumed to be due to the increased

due to the loss of bounded water molecules ].

The second decrease of the conductivity after 383 K (Fig. 6.27) is associated

with the loss of the dopant ion from

The activation energy was calcu r the linear part of the obtained

dependence (Table 6.6). As one can see from

activation energy are similar, except one refore, we can

assume that the use of organic acids to replace small molecule

d

, the acc

localization of the electronic wave function

[77, 169

the PM, i.e. with the dedoping process.

lated only fo

the obtained results, the values of the

for DBSA-doped PANI. The

201


Fi site

powder doped by different acids at 303 K

gure 6.26. Frequency dependence of real part of conductivity for the compo

PA-12/PANI

Figure 6.27. Arrhenius plot for the doped PA-12/PANI powder

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108

σ', S

/cm

10-3

10-2

HCl H2SO4

TSADBSA

1000/T, K-1

σ', S

/cm

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.010-3

10-2

HClH2SO4

TSADBSA

202


acids as a dopant may enhance the thermal stability of protonated PANI because the

evaporation of the dopants may be restricted.

Table 6.6. The activation energy for the doped PA-12/PANI powders


HCl 0.053

H2SO4 0.053

TSA 0.048

DBSA 0.061

6.3.2. Influence of the polyaniline content

One of the most remarkable effects in the conducting composite materials is

the percolation behaviour of the conductivity as a function of the PANI weight fraction

[256]. Such investigation will provide information on the influence of the PANI content

on the electrical and dielectric properties of the composite materials and will allow

estimating the percolation threshold (pc) for the conducting composite materials.

The percolation theory is based on the formation of the conducting network in

the insulating PM. For example, it was shown [221] that complete miscibility of the

conducting particles with insulating matrix is not desirable as in this case the conducting

network will not be created. In the case of the polymer composites filled with the

conducting polymers, the percolation threshold pc is defined as the minimum amount of

the conducting filler which must be added to an insulating matrix to cause the onset of

the electrical conductivity. This sudden jump in the conductivity is attributed to the

formation of the first “infinite” agglomerate pathway that allows electrons to travel

through a microscopic distance in the composite [256]. According to the theory [256],

this occurs when the filler represents 16 vol.% in the mixture. It means that below this

concentration the composites are very resistant to electrical flow, whereas above this

value the composites are conductive.

Relaxation properties. The frequency-dependent real conductivity σ’ at

various PANI-TSA contents (p) is presented in Fig. 6.28 at 303 K. It should be noticed

that the PA particles used in this study were monodisperse (5 µm).

203


204

sion

]. This

he increase of the conductivity with the frequency is due to the presence of

various kinds of inhomogeneity in the composite materials [258]. The poor frequency

dependence of the conductivity at higher PANI-TSA contents indicates that more

homogeneous materials are produced. Dutta et al [172], who investigated ac-

conductivity of PANI doped with NSA at various doping concentrations, found similar

behaviour, i.e. the frequency dependence of the conductivity is much more noticeable

w

The complex permitt NI-TSA contents in the

PA-12 matrix are depicted in Fig. 6.29. It is clearly observed that the increase of the

Figure 6.28. The frequency dependence of the real part of the complex conductivity for

the different PANI-TSA weight fractions at 303 K

The usual behaviour of disordered conductors is recovered: conductivity

remains constant at low frequency up to some onset frequency ωc where it starts to

increase. This frequency is the so-called crossover frequency ωc, defined as the

frequency at which ac-conductivity differs from the dc-plateau [257]. For charge carrier

diffusion on percolation structures, ωc is related to the transition from normal diffu

(σ’ (p > pc) = σdc = const) to anomalous diffusion (σ’ (p < pc) ∼ ωs) [257

crossover frequency increases with the increasing of the PANI content (Table 6.7).

T

ith the NSA concentration decreasing.

ivity spectra for the different PA

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107

σ',S

/cm

10-8

10-7

10-6

10-5

10-4

10-3

10-2

0.48 wt.% PANI-TSA1.59 wt.% PANI-TSA3.19 wt.% PANI-TSA5.04 wt.% PANI-TSA7.53 wt.% PANI-TSA9.92 wt.% PANI-TSA


Figure 6.29. Th rts of

the complex permittivity for the d at 303 K: (ο) 0.48 wt.%;

( ) 1.59 wt.%; (◊) 3.19 wt.%

Frequency, Hz

10-2 10-1 100 101 102 103 104 105 106 107 108

ε"

1

10

1

1

1

10

106

107

108

5

04

03

02

1

00

e frequency dependence of the real ε’ (a) and imaginary ε” (b) pa

ifferent PANI-TSA content

Frequency, Hz101 102 103 104 105 106 107 108

ε'

0

200

400

600 (a)

(b)

205


Table 6.7. dc-Conductivity and crossover frequency for the composite

PA-12/PANI-TSA powders

PANI-TSA content, wt.% σdc, S/cm ωc, Hz

0 ∼10-14

0.48 9.89⋅10-8 3.96⋅102

1.59 1.49⋅10-6 4.51⋅103

3.19 4.57⋅10-5 1.73⋅105

5.04 2.07⋅10-4 5.85⋅105

7.53 6.43⋅10-4 1.32⋅106

9.92 1.02⋅10-3 4.44⋅106

Pure PANI-TSA 4.905⋅10-1

PANI-TSA content leads to the increase of the relaxation frequency and the static

permittivity up to a certain value. For the PANI-TSA content higher than 3.19 wt.% it

was not possible to measure reasonable values of ε*.

From Fig. 6.29a we observe that a high value of permittivity has been found.

This may be due to an easy charge transfer through well ordered polymer chains in

disordered regions as suggested by Joo et al [259]. This behaviour has been observed in

unblended PANI-HCl by Zuo et al [168].

The power law behaviour of ε”(ω) for frequency lower than some critical

frequency is directly related to the dc-conductivity. A significant difference between the

two groups is also visible in dc-conductivity (σdc = σ’ (ω→0) (Fig. 6.29), which is a

measure of the long range movements of the charge carriers.

We have studied the frequency dependence of the dielectric constant ε’ and ac-

conductivity at several temperatures for the composite material with 3.19 wt.% of

PANI-TSA as shown in Fig.6.30. It is clearly observed that the relaxation frequency

increases with the increase of temperature. The temperature dependence of the

relaxation time τ, calculated from HN function (equation (2.7)), is depicted in Fig. 6.31.

It follows an Arrhenius law (equation (2.8)), which indicates a thermally activated

behaviour of the observed relaxation process. The best fit are obtained with 3⋅10-9 s and

0.067 eV for τ0 and the activation energy, respectively.

On the other hand, the conductivity dependence of the PA-12/PANI-TSA

206


Frequency, Hz102 103 104 105 106 107 108

ε'

0

50

100

150

200

25

300

0

173 K193 K(a) 213 K 233 K 253 K273 K 293 K HN fit

Figure 6.30. The real permittivity (a) and real conductivity (b) for the PA-12/PANI-

TSA (3.19 wt.%) composite as a function of frequency at different temperatures

Frequency, Hz100 101 102 103 104 105 106 107 108

σ', S

/cm

10-6

10-5

10-4

10-3

10-2

173 K193 K213 K 233 K253 K 273 K 313 K 353 K 373 K 393 K

(b)

207


Figure 6.31. Temperature variation of relaxation time of the PA-12/PANI-TSA (3.19

ma erial (Fig. 6.30b) is well described by the general power law:

, (6.3)

here σdc is the dc-conductivity, A is a temperature dependent constant and s is the

requency exponent ranging from 0 to 1 [260]. The second item in equation (6.3)

epresents the frequency dependent conductivity known as ac-conductivity.

The value of s for various temperatures has been determined from the linear

lope of log σac(ω) versus log ω. The value of s was found to increase with the decrease

f temperature as exhibited in Fig.6.32 and to lie between 0.25 and 0.84. The value of s

hopping conduction in amorphous materials [260].

Conductivity.

wt.%) powder. Solid line is a fit to Arrhenius law (equation (2.8))

ts

dc A ωσσ ⋅+=

w

f

r

s

o

is in accordance with the theory of

The dependence of the dc-conductivity on the PANI content (p)

ear the percolation threshold pc can be described by the power law [256]:

, (6.4)

here σ0 is a constant, t is a critical exponent that depends on the dimension, p is the

eight fraction of the filled particles, and p is the percolation threshold.

The extrapolated values of the dc-c

composite powders with the different PANI contents are illustrated in Fig. 6.33 for two

nt

cdc pp )(0 −= σσ

w

w c

onductivity variations at 303 K of the

1000/T, K-13.5 4.0 4.5 5.0 5.5 6.0

rela

xatio

n tim

e τ,

s

10-8

10-7

10-6

208


Figure 6.32. Variation of s with temperature for the PA-12/PANI-TSA (3.19 wt.%)

composite

dopants: TSA and DBSA. It is clearly seen that the electrical conductivity

systematically increases with the PANI weight fraction in PA. But the conductivity of

the bulk conducting composites that were synthesized using TSA as doping acid was

about one order of magnitude higher than that synthesized using DBSA. This can be

attributed to the influence of the counter anion. The DBSA anion has a rather long alkyl

chain and in order to be incorporated in PANI as a counter anion a sufficient place is

necessary. Therefore, the PANI chains will have to deform, resulting in a stressed

polymer backbone and leading to a decrease in conductivity [261]. Nevertheless, the

powders exhibit reasonably good conductivity even at a very low PANI content. Thus,

at the lowest content used, namely, 0.48 wt.% of PANI-TSA, the conductivity is

1.96⋅10-6 S/cm. But the value of conductivity for the PA-12/PANI-TSA composite (9.92

wt.%) is sm

shown in T

uction of the

conjugation length in the PANI chains. Similar results were obtained for other PANI

aller by 2 orders of magnitude in comparison with pure PANI-TSA as

able 6.7. On one hand, this may be because the PA particles hinder carrier

transport between different molecular chains of PANI and, on the other hand, an

interaction at the interface PA particles/PANI also leads to the red

Temperature, K150 200 250 300 350 400 450

s

0.8

0.4

0.5

0.6

0.7

0.3

0.2

0.9

209


Figure 6.33. Dependence of the dc-conductivity on the PANI-acid weight fraction p at

303 K. The solid lines are fit to equation (6.4). The inset shows the values of σdc above

the perc

tion

of the P

e fitted the σdc data presented in the Fig.6.33 to

equation

olation threshold versus log (p – pc) where the solid lines correspond to the best

fitted line

composites based on common insulating polymers. For example, Chattopadhyay et al

[262] systematically studied the variation of conductivity values at 303 K for different

methyl cellulose (MC)/PANI composites in the alcohol medium. They observed that the

value of conductivity decreases with the increase of alcohol and MC concentrations.

When conductivities of the PA-12/PANI-acid powders are plotted as a func

ANI content in a semi-logarithmic scale, a sharp change in conductivity is

observed at ∼1 wt.% of PANI-acid complex as shown in Fig. 6.33. At this composition

the conductivity changes significantly (over more than 5 decades). This point, probably,

corresponds to the percolation threshold (pc). It is known that the percolation threshold

depends upon the shape of the distribution of conductive particles in the matrix polymer

[263] and, also, on the nature of the acid-dopant [264]. In order to estimate the precise

value of pc and the critical exponent t, w

(6.4) and the best-fit values giving the correlation coefficient of 0.98, are

PANI-acid content, wt.%0 2 4 6 8 10

σ dc, S

/cm

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

TSADBSA

(p-pc)10-2 10-1 100 101 102

σ dc, S

/cm

10-6

10-5

10-4

10-3

10-2

210


presented in Table 6.8. The obtained low values of the percolation threshold are similar

t observed for the cellulose acetate/PANI blends [265]. We also found that the

percolation threshold pc is temperature-independent.

Table 6.8. The best-fit percolation parameters for the PA-12/PANI-TSA and PA-

12/PANI-DBSA powders

Acid-dopant Percolation threshold pc, wt.% σ0, S/cm t

to tha

TSA 0.37 5⋅10-5 1.85

DBSA 0.61 1.94⋅10-5 1.51

The fit was done by variation of pc in the interval from 0 to 1 in steps of

value of pc the value of t has been determined from the slope of the lin

relation of the dc-conductivity σdc and (p-pc) in a log-log scale (see inset for Fig. 6.

One can see in Fig. 6.33 that the agreement between the data and the theoretical la

qualitatively good on the whole range of composition while it is expected to be va

0.01.

For each ear

33).

w is

lid

only near the percolation threshold. The values of t (Table 6.8) calculated here for the

PA-12/PANI composites are in agreement with the theoretical value of t ≈2.0 for a

percolation network in three dimensions [256]. As one can see from the obtained results

(Table 6.8), in the case of PANI doped by TSA the parameter t is closer to the

theoretical value for the three dimensional network (∼2.0 [256]) than in the case of

doping by DBSA. The smaller value of t observed for the PA-12/PANI-DBSA powder

arises from induced hopping transport between disconnected (or weakly connected)

conducting parts of the network.

It should be mentioned that the obtained values for the percolation threshold

are rather low compared with those given in literature. For example, Tsotra and

Friedrich who studied the epoxy resin/PANI-DBSA blends [266] determined a

percolation value at about 2.5 wt.% of the conductive salt. Zhang et al [225] studied the

PA-11/PANI blends prepared by the dissolution of PA-11 and PANI in the sulphu

acid and established that the percolation in such a system occurs only at about 5 wt.%.

A

t

percolation threshold, which d into the composite with a

ric

t this value of the PANI content, the conductivity of the composite blend was more

han 7 orders of magnitude higher than that of pure PA-11 [225]. Lowering the

means that less PANI was adde

211


good conductive property, not only reduces the cost of the materials but also improves

its proce

Fig. 6.34 shows the ith temperature for the PA-

12/PANI-TS e esti ation en qual to 0.104

eV. The observed difference between the activ ies for the composite powders

based on PA at the sam NI content (2.3 wt.% counting on dedoped PANI) can be

explained by the different PA particle size – compare activation energy of 0.104 eV for

PA particle of 5 µm an 8 eV - for 14-24 us, with increasing of the size of

the PA particles the va the activation en creased. T rvation can be

explained by the fact that the effective surface which should be covered to provide the

percolation network increases with the decrease of the PA particle size.

vation energy (Table 6.9).

ssability.

variation of conductivity w

A (3.19 wt.%) sample. Th mated activ ergy is e

ation energ

e PA

d 0.04 3 µm. Th

lue of ergy is de his obse

10

-4

Figure 6.34. Conductivity of the PA-12/PANI-TSA (3.19 wt.%) sample as a function of

temperature

Thermal variations of electrical conductivity of the PA-12/PANI-acid powders

with different weight fractions of PANI were performed for powders doped with TSA

and with DBSA. A study of the temperature dependence of the dc-conductivity in the

range of 150-450 K allowed the determination of the acti

1000/T, K-1

3.0 3.5 4.0 4.5 5.0 5.5 6.0

σ', S

/cm

-5

10-6

10

212


Table 6.9. A comparison of the activation energy of the doped PA-12/PANI powders

Composition of the sample

Activation energy

Ea, eV

PA-12/PANI-TSA (0.48 wt.%) 0.230

PA-12/PANI-TSA (1.59 wt.%) 0.211

PA-12/PANI-TSA (3.19 wt.%) 0.104

PA-12/PANI-TSA (5.04 wt.%) 0.089

PA-12/PANI-TSA (7.53 wt.%) 0.086

PA-12/PANI-TSA (9.92 wt.%) 0.068

PA-12/PANI-DBSA (0.61 wt.%) 0.258

PA-12/PANI-DBSA (1.13 wt.%) 0.226

PA-12/PANI-DBSA (3.61 wt.%) 0.154

As one can see from the obtained results, the activation energy decreases with

the PAN content increasing, indicating the facilitation of the charge transport which

could be attributed to the formation of new conducting paths of PANI as observed for

PANI with different protonation levels doped by HCl [227, 267] and H2SO4 [268]. Such

feature confirms that as far as the PANI content increases, the conductivity of the

composite increases too.

he rapid increase of the activation energy with the decrease of the PANI

fraction reveals the change of geometry which occurs below the percolation threshold

with the rupture of conducting paths. Similar behaviour was observed by Jousseaume et

al [269] for the polystyrene/PANI blends doped by CSA and bis(2-ethyl-hexyl)

hydrogen phosphate.

nfluence of additional doping.

I

T

I The influence of additional doping on the

electrical properties of the composite materials with different PANI-TSA contents has

also been investigated. In Table 6.10 the results of the dc-conductivity measurements

demonstrate the change of the value of the percolation threshold and the value of the

parameter t.

a result of additional doping the value of t is closer to 2, i.e.

the percolation network occurs in three dimensions. Also, this result confirms the data

of Rama spectrometry (see section 5.5.3) about the presence of the non-doped imine

It is found that as

n

213


Table 6.10. Influence of the additional doping on the percolation parame


Percolation threshold pc, wt.% σ0, S/cm

ters for the

t

As-prepared 0.37 5⋅10-5 1.85

After additional doping 0.3 7⋅10-5 1.95

sites in the PANI structure after the initial polymerization process. We can make such a

conclusion as the slight increase of the conductivity of the PA-12/PANI composites,

obtained by in situ dispersion polymerization in the presence of TSA is observed due to

th

Figure 6.35. Electrical conductivity of the PA-12/PANI-TSA composites as a function

of the PANI-TSA content and doping treatment at 303 K

It is evident that the treatment with additional TSA doping enhances slightly

the conductivity of the composites and reduces a little the percolation threshold.

The temperature dependence of conductivity was also performed in the case of

the additionally doped PA-12/PANI-TSA powders and Table 6.11 shows that the values

of the activation energy are also decreased with the increase of the PANI content. The

e additional doping (Fig. 6.35).

PANI-TSA content, wt.%0 2 4 6 8 10 12

σ dc, S

/cm

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

without additional dopingwith additional doping

214


Table 6.11. The activation energy for the PA-12/PANI-TSA powders after additional

doping

Composition of the sample

Activation

energy Ea, eV

PA-12/PANI-TSA (0.48 wt.%) 0.162

PA-12/PANI-TSA (1.59 wt.%) 0.156

PA-12/PANI-TSA (3.19 wt.%) 0.117

PA-12/PANI-TSA (5.04 wt.%) 0.088

PA-12/PANI-TSA (7.53 wt.%) 0.087

PA-12/PANI-TSA (9.92 wt.%) 0.074

smaller values of the activation energy for the samples after the additional doping imply

a longer decay length and a higher density of states at the Fermi level (see equations

(1

6.3.3. Influence of the polyamide matrix structure

.13) – (1.15)).

In order to reveal the influence of the PM on the conductive properties of the

bulk composite materials, the PA-11 matrix was a

rather high

conducti

0.031 eV and 0.041 eV for the composites doped with TSA and HCl, respectively.

Analyzin

, because of

the similar matrix molecular structure and close c

this case, as in

the case

lso used for the preparation of the

bulk PA-11/PANI composite materials. The PA particle size was 14-42 µm.

The increase of the complex permittivity values after the doping process for the

composites based on PA-11 was also observed, but because of a

vity value no relaxation process was observed in low-frequency region. As in

the case of the PA-12/PANI-acid composites, at high frequencies a relaxation process

was observed. The relaxation times τ at different temperatures are determined and fitted

with an Arrhenius law (equation (2.8)). The best-fit values for the activation energy are

g the obtained results we may conclude that for both PM (PA-11 and PA-12)

the parameters of the observed relaxation are practically similar, probably

onductivity values.

In Fig. 6.36 the conductivity for the PA-11/PANI composite powders doped

with different acids is plotted at 303 K. The figure clearly shows that in

of the bulk composite based on PA-12, a dc-conductivity plateau is observed

for all dopants. The value of conductivity for the PA-11/PANI-TSA composite is

215


PA-11/PANI powd ith different acids

Figure 6.36. The conductivity dependence as a function of frequency for the composite

ers doped w

slightly higher than that of the PA-12/PANI-TSA composite (9⋅10-3 S/cm and 6⋅10-3

S/cm, respectively). As for the surface conducting composites (see section 6.2), a higher

conductivity is observed for the composites based on PA with a lower (CH2) groups

concentration. Therefore, we can conclude that the electrical properties of the

composites depend on the structure of the PM.

Measurements of conductivity as a function of temperature were also

performed and it was found that the composite conductivity increases with temperature

up to a certain value for all used acids. A linear relationship was observed at T < 293 K.

Two peaks of conductivity were observed on the received curves – the first one at 343

K and the second one – at 383 K, after which the final decrease of conductivity

occurred. The calculated activation energy values are presented in Table 6.12.

6.3.4. Properties of the composite films

The possibility of producing the conducting composite PA/PANI films can be

an advantage when industrial applications are considered. That’s why the composite

films corresponding to the previously studied powders were obtained by the

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108

σ', S

/cm

10-12

10-11

10-10

10-9

10-810-3

10-2

HCl H2SO4 TSADBSADedoped

216


compression- molding technique from the powder samples at 195 0C (a detailed

description of the film preparation is given in section 2.2.2).

Table 6.12. The activation energy for the PA-11/PANI samples doped with different

acids


HCl 0.183

H2SO4 0.161

TSA 0.072

DBSA 0.097

Influence of the dopant nature. In order to display the changes in the

conductivity value we performed measurements of the composite materials in the film

. Fig. 6.37 shows the frequency dependence of the conductivity for the Pform A-

2/PANI composite films doped by different acids measured at 303 K. The spectra of

t

c

difference as compared to pure PA-12 spectra of the doped composite films

we observed a frequency-independent conductivity region at lower frequencies. This

1

he pure and dedoped samples are given for comparison. The real part of the

onductivity of pure PA-12 increases with increasing the frequency and at 0.1 Hz gives

value of ∼10-14 S/cm. The spectrum of the dedoped composite film does not reveal any

while in the

conductivity part is identified as the dc-conductivity. Above a characteristic onset

frequency ωc, the conductivity increases with frequency (Fig. 6.37). The ωc value

increases with increasing the conductivity and it is 5.1⋅104 Hz, 7.7⋅104 Hz and 3.9⋅105

Hz for the composites doped by HCl, DBSA and TSA, respectively.

The conductivity of the composite PA-12/PANI-acid films obey Arrhenius law

and the activation energy is found to be 0.077 eV, 0.078 eV and 0.103 eV for the films

doped by TSA, HCl and DBSA, correspondingly.

It should be mentioned that for the composite materials in the powder form at

303 K only dc-conductivity is observed in the frequency range 0.1 Hz-1MHz (Fig.

6.26). dc-Conductivity value of the films is lower than that of the corresponding powder

composite. For example, for the PA-12/PANI-TSA composite in the powder form the

conductivity is 6⋅10-3 S/cm, while for the corresponding sample in the film form – only

217


e film preparation we were obliged to stop at 195 0C.

During t

Figure 6.37. Frequency dependence of the conductivity for the composite

PA-12/PANI-acid films at 303 K

1⋅10-6 S/cm. But, on the other hand, the acid nature dependence remains the same in

both sample forms – the most conductive is the sample doped by TSA (Fig. 6.26 and

6.37).

The observed differences in the conductivity values for the composite samples

in powder and film forms may be explained by the thermal instability of the PANI

conducting complex as during th

his procedure the partial dedoping process takes place and a decrease of the

conductivity is afterwards observed. In addition, this high temperature may cause the

disorganization of PANI in the insulating matrix (the derangement of the percolation

net).

Influence of the PANI content. It was shown that the dopant nature has a great

influence on the electrical properties of the composite materials. In order to reveal the

influence of the conducing complex content on the composite conductivity we

performed measurements for the materials in the film form.

Frequency, Hz10-2 10-1 100 01 102 103 104 105 106 107 1081

σ', S

/cm

10-6

10-5

10-14

10-13

10-12

10-1

10-10

10-8

-7

1

10-9

10

PA-12/PANI-HCl PA-12/PANI-TSAPA-12/PANI-DBSAPA-12PA-12/PANI-dedoped

ωc

ωc

218


The changes of the conductivity with the PANI contents are illustrated in Fig.

6.38. Apparently, the conductivity increased dramatically for the PA-12/PANI-TSA and

PA-12/P

Figure 6.38. The dependence of the dc-conductivity of the composite PA-12/PANI films

(pressing temperature 195 0C) at 303 K

When the PANI content approaches the percolation value, the conductivity

rapidly increases by some orders of magnitude and then increased gradually with further

increase of the PANI content in the system. The acid-dopant also affected the

percolation threshold as can be seen from Table 6.13.

as it was established (see section 5.3, Fig. 5.8), resulted in a small decrease of the tensile

strength. Therefore, introducing of PAN , on one hand, improves the electrical

conducti

both

used acids (Fig. 6.33), but for the samples in the film form they are

ANI-DBSA composite films, when the PANI content approaches the

percolation threshold.

PANI-acid content, wt.%0 2 4 6 8 10 12

σ dc, S

/

10-12

10-11

10-10

10-9

cm

The increase of the content of the conductive PANI complex in the composites,

I

vity of the composites, but, on the other hand, it has a negative influence on

mechanical properties of the composite materials.

As it was established for the samples in the powder form the value of

conductivity and its dependence on the PANI content have the same behaviour for

10-8

10-7

10-6

TSADBSA

219


PA-12/PANI films

reshold pc, σ0, S/cm t

Table 6.13. Influence of the dopant on the percolation parameters for the composite

Percolation th wt.%

PA-12/PANI 4.52 2.08⋅10-9 1.40 -TSA (195 0C)

PA-12/PANI- 5.63 1.72⋅10-8 0.43 DBSA (195 0C)

dependent on hown by G t al [221] that the

completely mi k. Probably, in the

case of using takes place and, thus, the percolation

network is for small valu rameter t (0.43) also

testifies to this

It is a ent with additional doping leads to the

ecrease of the percolation threshold to 3.03 wt.% in the case of TSA-doped PANI. But

addition

r up to 2 wt.% [158]) this can lead to the acidic hydrolysis.

Thus, th

s of samples are obtained with different frequency-dependences,

which are in good agreem

twork can not be formed. As the PANI

(for example, the sample with 3.19 wt.%

of PANI-TSA) the appearance of

the acid (Fig. 6.38). It was s azotti e

scible composites do not form the percolation networ

DBSA such “ideal dissolution”

med with difficulty and later. The e of pa

fact.

lso necessary to note that the treatm

d

al doping, on the other hand, results in a slight decrease of the conductivity

value – for example, the conductivity value for 9.92 wt.% of PANI-TSA is 1.86⋅10-7

S/cm before and 4.9⋅10-8 S/cm after additional doping. This can be explained by the fact

that during such treatment the additional quantity of the dopant (TSA in this case) is

introduced inside the PM. In the presence of the scanty quantity of water (it is known

that PA can absorb wate

e percolation network can be broken. The decrease of the tensile strength of the

film after additional doping (see Fig. 5.9) also testifies to this.

The measurements of conductivity as a function of frequency were performed

for both the PA-12/PANI-TSA (Fig. 6.39a) and PA-12/PANI-DBSA (Fig. 6.39b)

composites. Two group

ent with the results of percolation threshold (Fig. 6.38). Thus,

for samples with a low PANI-acid content the conductivity is frequency-dependent in

all studied frequency region. This can be explained by the fact that below percolation

threshold the conducting PANI particles are well dispersed within the insulating non-

conducting PA phase, i.e. the percolation ne

content approaches the percolation threshold

a linear part, i.e. the frequency-independent part of

conductivity, is observed (Fig. 6.39a). Therefore, above the percolation threshold the

composite films reveal a dc-conductivity, the value of which corresponds to the flat part

220


Figure 6

.39. The frequency dependence of the real part σ’ of the complex conductivity

for the PA-12/PANI-acid films with the different PANI-acid content at 303 K

Frequency, Hz10 07 108-2 10-1 100 101 102 103 104 105 106 1

σ', S

/cm

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

0.61 % PANI-DBSA1.13 % PANI-DBSA3.61 % PANI-DBSA5.64 % PANI-DBSA7.75 % PANI-DBSA

Frequency, Hz10-2 10-1 100 101 102 103 104 105 106 107 108

σ', S

/cm

10-14

10-13

10-8

10-5

10-4

10-3

10-7

10-6

10-12

10-11

10-10

10-9

0.48 % PANI-TSA1.59 % PANI-TSA3.19 % PANI-TSA5.04 % PANI-TSA9.92 % PANI-TSA

(a)

(b)

221


of the frequency dependence.

t

measurement for the composite PA-12/ A (3.19 wt.%) film pressed at 195 0C

as a function of frequency and temperature simultaneous

real part of permittiv 40. It ved that with the

temperature incr fted to the requency region.

.

to equation (2.7)

lained by the fact

that both

In order to reveal the nature of the relaxation process we carried ou

PANI-TS

ly. The obtained results of the

is clearly obserity ε’ are depicted in Fig. 6.

ease the relaxation process is shi higher f

Figure 6.40. The frequency dependence of the real part of dielectric permittivity for the

PA-12/PANI-TSA (3.19 wt.%) film at different temperatures. Solid lines are the best fit

The relaxation process calculated according to HN function (equation (2.7)) is

characterized by an activation energy of 0.087 eV and a symmetric distribution of

relaxation time (α ~0.5, β = 1). A correlation between the relaxation time and

conductivity were found. The dc-conductivity and the relaxation frequency are both

thermally activated with the same activation energy, which may be exp

5

8

9

10

processes originated from the same transport mechanism.

Thermal variations of electrical conductivity for the PA-12/PANI films with

different weight fractions of the PANI conducting complex are shown in Fig. 6.41a - for

films doped with TSA and in Fig. 6.41b - for films doped with DBSA. In both cases the

Frequency, Hz

10-2 10-1 100 101 102 103 104 105 106 107 108

6

7

ε'

3

4

173 K193 K 213 K233 K 253 K 273KHN fit

222


Fig n of temperature for the composite

10-5

10

ure 6.41. Electrical conductivity as a functio

PA-12/PANI films doped with (a) TSA and (b) DBSA

Temperature, K150 200 250 300 350 400 450

σ dc, S

/cm

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

-4

0.61 % PANI-DBSA 1.13 % PANI-DBSA 3.61 % PANI-DBSA 5.64 % PANI-DBSA 7.75 % PANI-DBSA

(b)

Temperature, K150 200 250 300 350 400 450

σ dc,

S/cm

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

0.49 % PANI-TSA 1.59 % PANI-TSA 3.19 % PANI-TSA 5.04 % PANI-TSA7.53 % PANI-TSA

(a)

223


plots show that two different regions could be clearly distinguished. The presence of the

two regions on the temperature dependence is also observed in the cas

additionally doped by TSA and for films pressed at higher temperatures (220

24

e of films 0C and

e

conductivity value rapidly and significantly increases. One may suppose that the

presence of this transition is connected w mperature Tg of the

PA film as the value of PA-12 is 37 0C . After this temperature the

movements of the polym omolecules becom er and, as a result, the ionic

conductivity increases.

Activation energy values for the compos s with different PANI-acid

contents are estimated and listed in Table 6.14. The obtained values reveal, as in the

case of

0 0C). The transition between these regions is observed at 310 K, after this point th

ith the glass transition te

Tg for [190]

er macr e easi

ite film

the samples in the powder form, that the activation energy decreases as the

PANI content increases, or, in other words, conductivity increases suggesting the

formation of more polarons responsible for electrical conduction in PANI. However, for

the composites in the film form the values of the activation energy are much higher than

those for the composites in the powder form (compare Table 6.9 and 6.14).

Table 6.14. The activation energy as a function of the PANI-acid content in the

PA-12/PANI composite films

Composition of the sample Activation energy Ea, eV

PA-12/PANI-TSA (0.49 wt.%) 0.688

PA-12/PANI-TSA (1.59 wt.%) 0.656

PA-12/PANI-TSA (3.19 wt.%) 0.563

PA-12/PANI-TSA (5.04 wt.%) 0.410

PA-12/PANI-TSA (7.53 wt.%) 0.135

PA-12/PANI-DBSA (0.61 wt.%) 0.891

PA-12/PANI-DBSA (1.13 wt.%) 0.891

PA-12/PANI-DBSA (3.61 wt.%) 0.874

PA-12/PANI-DBSA (5.64 wt.%) 0.787

PA-12/PANI-DBSA (7.75 wt.%) 0.644

higher than that of TSA doping, which is in agreement with the value of the percolation

It should be noted that the activation energy in the case of DBSA doping is

threshold.

224


In order to reveal the thermal stability of conductivity for the synthesized PA-

12/PANI composite films, the conductivity of the film with 9.92 wt.% of PANI-TSA

was recorded as a function of time at 423 K (Fig. 6.42). The obtained plot exhibits a

downward tendency as the ageing proceeds. Wolter et al [270] assumed that the

the

ent can

a

So, the obtained result shows that the conductivity after a heat treatment at 423

K durin

deprotonation of a conducting polymer, which is responsible for the loss of conductivity

value, is controlled by a diffusion-like process that starts at the surface of

conducting grains. Furthermore, it has been postulated [271] that a heat treatm

increase the concentration of structural defects in the film, which also results in

decrease of the charge mobility.

Figure 6.42. The time dependence of the conductivity for the PA-12/PANI-TSA (9.92

wt.%) composite films at 423 K

g 2.5 h decreased by less than one order of magnitude. Therefore, these

conducting films are rather promising materials used for producing, for example,

photographic films or plastic foils.

Influence of the polymer matrix structure. The influence of the PM structure

was also investigated by the example of PA-11. The observed value of the onset

Time, min

0 20 40 60 80 100 120 140 160

σ dc, S

/cm

10-5

10-4

225


frequenc

composite PA-11/PANI-acid films obey Arrhenius law and the

activation energy is found to be 0.064 eV and 0.099 eV for the films doped by TSA and

HCl, correspondingly. The obtained values are of the same order of magnitude as for

the composite films based on the PA-12 film.

Fig

6.4.

rpreted. The obtained results can be

summari

y ωc increases with increasing conductivity of the composite film (Fig. 6.43) as

in the case of PA-12. So, the highest value of ωc is for the film doped with TSA (1.5⋅104

Hz) and the lowest one is for the DBSA doped film (52 Hz). It was also found that the

obtained results for the

ure 6.43. The frequency dependence of the conductivity for the composite

PA-11/PANI films at 303 K

Conclusions

The temperature and frequency dependences of the permittivity and

conductivity for pure PANI as well as for the surface and bulk conducting composite

PA/PANI materials have been measured and inte

zed as follows:

Frequency, Hz10 10 10 10

σ', S

/cm

10

10-14-2 -1 100 101 102 103 104 105 106 7 8

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

HCl TSADBSADedoped

226


1. High permittivity value (∼100) is found for dedoped PANI due to the

pr r

dedoped PANI and is attributed to the conductivity relaxation, since a correlation of the

conductivity with the relaxation tim

ments for the d PANI samples show a significant

i value. Nevert , a relaxation process at high

frequencies is observed. Besides, the position of this relaxation is found to be dependent

l relaxation processes are found in the doped composite related to

its cond

wt.%

esence of charge carriers. Also, a relaxation process is found at low frequencies fo

e was found.

2. Performed measure oped

ncrease of the conductivity heless

on the acid-dopant used, which is explained by the different counter-anion size and the

different PANI conformation.

3. For the surface conductive composite PA/PANI materials besides the

subglass relaxation processes (β and γ) occurring in the PM as well as in the dedoped

composite, additiona

uctive properties. Interfacial polarization relaxations in the MHz region are

attributed to the layered and clustered structure of the composite. At higher frequency,

conductivity relaxation appears to be connected with the interfacial polarization in the

PANI clusters. The structure of the PM is found to influence these relaxations.

4. Although the surface conductivity value of the composite films can not be

measured using DRS, the conductive properties of these films have been evaluated

through the study of the interfacial relaxation processes in the films.

5. The conductivity of the bulk conductive composite either in powder or in

film forms found to be dependent on the acid-dopant and on the PANI content. A low

percolation threshold is found for the PANI-TSA and PANI-DBSA complexes (0.37

and 0.61 wt.%, respectively) in the powder form whereas in the film form it

increases to 4.52 wt.% and 5.63 wt.%, correspondingly, for the PA/PANI-TSA and PA-

PANI-DBSA composites.

6. The influence of the PANI content on the frequency dependence of the

permittivity and conductivity revealed that the relaxation frequency, attributed to

interfacial polarization effect, shifted to higher frequency region and the onset

frequency of the conductivity also increased with the increase of the PANI content.

7. The activation energy was found to decrease with the increase of the PANI

content as well as after the treatment by additional doping.

8. The increase of the conducting complex content in the composite leads to the

increase of the composite conductivity value in the powder form as well as in the

pressed film form. However, it was found that the conductivity of the samples drops to a

227


very low value when they are pressed to form films. This can be explained by the

influence of the high temperature value at which films are prepared.

228

General conclusions and

suggestions for future work

General conclusions and suggestions for future work

In the present study, conducting polymer composite materials based on PANI

and PA, either surface or bulk, in powder or film forms, were prepared by the in situ

electrochemical and chemical polymerization methods. The properties of the obtained

materials were investigated in detail by a set of physico-chemical methods, such as

DRS, Raman and UV-Vis spectroscopies, thermogravimetry, spectroelectrochemistry,

tensile strength and conductivity measurements, etc. For comparison, pure PANI was

prepared by the electropolymerization and by chemical polymerization methods. Its

thermal, electrical and dielectric properties in both doped and dedoped states were

investigated to allow a better understanding of the studied properties of the composites.

The peculiarity and kinetics of the chemical and electrochemical aniline

polymerization in the polymer matrix revealed that either in the polymer film or in the

presence of the polymer dispersion the aniline polymerization process runs according to

the typical autocatalytic mechanism. Three stages – an induction period, a propagation

stage and a termination stage – have been pointed out. The aniline polymerization

process in the polymer matrix has noticeably higher reaction orders with respect to

aniline and oxidant (APS) in comparison with the same process in the solution. The

efficiency of the aniline polymerization process depends on the matrix’s ability to swell

in the oxidant solution, on the oxidant and monomer diffusion into the reaction zone.

This is associated with the physicochemical interaction of aniline and PANI with the

PM (formation of hydrogen bonds).

To evaluate the real PANI content in the conducting composites a method

using UV-Vis spectroscopy has been developed.

It was shown that the dopant nature affects thermal properties of pure PANI.

The bulk conductive PA-12/PANI composites showed the increase by ∼100 0C of the

degradation temperature in comparison with pure PANI independently of the acid-

dopant used. Also, it was found that the presence of PANI in the PA matrix leads to a

decrease of its tensile strength. It was confirmed that the nature of the acid-dopant

influences the mechanical properties of the final composite film – the tensile strength is

higher when using the organic acid-dopants than when using the inorganic ones. Such

behaviour is connected with the fact that organic acids perform the triple role – of a

dopant, a plasticizer and compatibilizer.

Both surplus of the acid-dopant (which appeared after the treatment by

additional doping) in the PA/PANI composite and the increase of the pressing

temperature deteriorate the tensile strength of the final composite material. The main

229


reason of this phenomenon is the acidic hydrolysis of the PA matrix since the surplus of

the acid or its appearance in the material during the pressing at higher temperatures due

to dissociation of the conducting complex PANI-acid takes place.

The results of Resonance Raman spectrometry confirmed the interaction

between PANI and the PA matrix (hydrogen bonds). For the first time with the help of

Raman spectrometry the thickness of the formed PANI containing layer, which depends

on the PM structure, have been determined. In the case of the surface conductive

composite PA-6/PANI film the characteristic PANI bands were found to be more

intense than those of PA-12/PANI film due to the higher thickness of the PANI

containing layer and more quantity of formed PANI in the case of the PA-6 film. The

differences may be explained in terms of the PM molecular structure, which results in

higher swelling capacity of PA-6.

The layered and clustered structure of the surface conductive composites

revealed by AFM and optical microscopy and the core-shell organization (PA grains-

PANI) of the bulk conductive composites revealed by Raman spectrometry resulted in

the appearance of an interfacial polarization relaxation process. The relaxation

frequency of this process is found to be proportional to the dc-conductivity. These two

physical properties (relaxation process and conductivity) are found to be dependent

upon the dopant nature, the PANI content and the matrix structure.

In surface conductive PA/PANI composites, besides the subglass relaxation

processes (β and γ) occurring in the pure matrix and in the dedoped PA/PANI

composites, additional relaxation processes are found in the doped composite related to

its conductive properties. Interfacial polarization relaxations in the MHz region are

attributed to the layered and clustered structure of the composite. At higher frequency,

conductivity relaxation appears to be connected with the interfacial polarization in the

PANI clusters.

The study of the conductivity dependence on the PANI-acid complex content

for the bulk composites revealed a low percolation threshold for the composites in

powder form - 0.37 wt.% and 0.61 wt.% for the PANI-TSA and PANI-DBSA

complexes, respectively. However, in compression molded films the value of the

percolation threshold increases (4.52 wt.% and 5.63 wt.% for the PA/PANI-TSA and

PA-PANI-DBSA composites, correspondingly).

Therefore, in the obtained composites the insulating polymer (PA) provides

good mechanical, thermal properties and processability while the conducting polymer

230


(PANI) provides electrical conductivity. Obtained results testify to the practical and

scientific possibility of using these composite materials based on PANI and polyamide

as antistatic coatings and in sensors.

However, to improve the electrical properties of the compression molded film

and to enhance the thermal stability of the conductivity it would be useful to find (or to

synthesize) dopants, which will display higher thermal stability than those used in this

study.

Also, the study of the estimation of the PANI doping level in the PM/PANI

composites would be very effective and useful to determine the minimum necessary

amount of the doping acid leading to a high conductivity value of the final composite.

Finally, attention should be paid also to the influence of the size of the polymer

matrix particles on the composite properties. A full understanding of the behaviour of

this class of materials will be of benefit to the applications of conducting polymer

composite materials in various fields.

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Abstract

Two types of composites based on polyaniline (PANI) and polyamide (PA) were

elaborated by oxidative polymerisation: surface conducting polymer films with a subsurface

conducting layer and bulk conducting polymers in powder and film forms. In order to

determine the synthesis-structure-properties relationship, different physico-chemical methods

of investigation (cyclic voltammetry, dielectric relaxation spectroscopy, thermogravimetry,

microscopy, Raman spectrometry,….) were used. It is shown that the rate of the aniline

polymerization process inside the polymer matrix is determined by the matrix’s ability to

swell in the reaction media and by the rate of oxidant and monomer diffusion into the reaction

zone. Three well defined polymerization stages (induction period, propagation stage and

termination stage) are observed and the kinetic parameters of the process are calculated, that

enable the control of the polymerization process. It is confirmed that independently from the

presence or absence of the polymer matrix (film or dispersed media) the mechanism of aniline

polymerization does not change and the polymerization process follows an autocatalytic

mechanism. The thermal and mechanical properties of the composites are improved as

compared with pure PANI. The degradation temperature of the composite is enhanced by

∼100°C. The use of organic acid-dopants leads to better mechanical characteristics due to

their triple role (of a dopant, compatibilizer and plasticizer) in comparison with inorganic

ones. The layered and clustered structure of the surface conducting composites is revealed by

atomic force microscopy (AFM) and optical microscopy. The thickness of the conducting

layer is found to be dependent on the polymer matrix structure. This organisation is traduced

by the appearance of interfacial polarisation relaxation processes related to the conductive

properties of the PANI clusters. The core-shell (PA grains-PANI) structure of the bulk

conducting composites, as demonstrated by Resonance Raman Spectrometry, results in a high

conductivity value with a low percolation threshold for the powder form of the composites.

An interfacial polarisation relaxation process related to the composite structure is pointed out

which relaxation frequency increases with increasing PANI content and, hence, with

conductivity. Therefore, the obtained composites owning the mechanical properties of the

matrix and the electrical properties of PANI are good candidates for practical application such

as antistatic materials, materials for capacitors, sensors, etc.

Key words: polyaniline, polyamide, composite, conductivity, dielectric relaxation,

percolation, kinetics of polymerization, voltammetry, Raman spectrometry, AFM.

Résumé: Parmi les conducteurs organiques, la polyaniline (PANI) est le polymère

qui présente une grande stabilité environnementale et une grande conductivité électrique.

Cependant, elle présente également des inconvénients tels que ses faibles propriétés

mécaniques qui représentent un verrou technologique pour de nombreuses applications. Pour

y remédier, une des méthodes consiste à les associer avec des polymères conventionnels pour

former des composites ayant à la fois les propriétés mécaniques de la matrice et les propriétés

électriques de PANI. Aussi, deux types de composites conducteurs à base de PANI et de

polyamide (PA) ont été élaborés: les films composites conducteurs en surface et les

composites conducteurs en volume sous forme de poudres et de films. La synthèse rationnelle

et la conception de ces matériaux nécessitent l’établissement de relation synthèse-structure-

propriétés.

L’étude électrochimique et chimique a montré que la vitesse de polymérisation de

l’aniline est déterminée par la capacité de gonflement de la matrice dans le milieu réactionnel

et par la vitesse de diffusion de l’oxydant et du monomère. Trois stades de polymérisation ont

été clairement identifiés pour lesquels les paramètres de la cinétique ont été calculés

permettant ainsi le contrôle du processus autocatalytique de polymérisation.

Les études thermique et mécanique ont montré que les propriétés de ces composites

sont voisines de celles du polymère hôte pour de faibles taux de PANI. La structure en double

couche des films conducteurs en surface, d’une part, et l’organisation de PANI sous forme

d’amas conducteurs dispersés dans un milieu isolant, d’autre part, ont été mises en évidence

par spectrométrie Raman et par microscopie. Cette organisation se traduit, sur les propriétés

diélectriques, par l’apparition de relaxations de polarisation d’interface dont les fréquences de

relaxation sont corrélées avec la conductivité de la surface du film. Il a été montré que la

percolation des amas conducteurs, qui détermine les propriétés électriques et diélectriques du

composite, dépend de la structure de la matrice.

Les composites conducteurs en volume sous forme de poudre, de part leur

structuration en “core-shell” (PA/PANI), présentent des conductivités voisines de celles de

PANI avec des seuils de percolation très bas (<1 wt.% ). La structure “core-shell” s’est

traduite par l’apparition d’une relaxation de polarisation d’interface dont la fréquence

augmente avec le taux de PANI et de ce fait avec la conductivité.

De part les propriétés électriques, thermiques et mécaniques obtenues, ces

composites présentent de grandes potentialités d’application dans différents domaines.

Mot-clés: Polyaniline, polyamide, composite, cinétique de polymérisation, percolation,

voltammétrie, spectrométrie Raman, microscopie, spectroscopie diélectrique, conductivité.

Spécialité: Physique de la Matière...

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